Flywheel energy storage (FES) works by accelerating a rotor (flywheel) to a very high speed and maintaining the energy in the system as rotational energy. When energy is extracted from the system, the flywheel's rotational speed is reduced as a consequence of the principle of conservation of energy; adding energy to the system correspondingly results in an increase in the speed of the flywheel.
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Most FES systems use electricity to accelerate and decelerate the flywheel, but devices that directly use mechanical energy are being developed.[why?][1]
Advanced FES systems have rotors made of high strength carbon-fiber composites, suspended by magnetic bearings, and spinning at speeds from 20,000 to over 50,000 rpm in a vacuum enclosure.[2] Such flywheels can come up to speed in a matter of minutes – reaching their energy capacity much more quickly than some other forms of storage.[2]
A typical system consists of a flywheel supported by rolling-element bearing connected to a motor–generator. The flywheel and sometimes motor–generator may be enclosed in a vacuum chamber to reduce friction and energy loss.
First-generation flywheel energy-storage systems use a large steel flywheel rotating on mechanical bearings. Newer systems use carbon-fiber composite rotors that have a higher tensile strength than steel and can store much more energy for the same mass.[3]
To reduce friction, magnetic bearings are sometimes used instead of mechanical bearings.
The expense of refrigeration led to the early dismissal of low-temperature superconductors for use in magnetic bearings. However, high-temperature superconductor (HTSC) bearings may be economical and could possibly extend the time energy could be stored economically.[4] Hybrid bearing systems are most likely to see use first. High-temperature superconductor bearings have historically had problems providing the lifting forces necessary for the larger designs but can easily provide a stabilizing force. Therefore, in hybrid bearings, permanent magnets support the load and high-temperature superconductors are used to stabilize it. The reason superconductors can work well stabilizing the load is because they are perfect diamagnets. If the rotor tries to drift off-center, a restoring force due to flux pinning restores it. This is known as the magnetic stiffness of the bearing. Rotational axis vibration can occur due to low stiffness and damping, which are inherent problems of superconducting magnets, preventing the use of completely superconducting magnetic bearings for flywheel applications.
Since flux pinning is an important factor for providing the stabilizing and lifting force, the HTSC can be made much more easily for flywheel energy storage than for other uses. HTSC powders can be formed into arbitrary shapes so long as flux pinning is strong. An ongoing challenge that has to be overcome before superconductors can provide the full lifting force for an FES system is finding a way to suppress the decrease of levitation force and the gradual fall of rotor during operation caused by the flux creep of the superconducting material.
Compared with other ways to store electricity, FES systems have long lifetimes (lasting decades with little or no maintenance;[2] full-cycle lifetimes quoted for flywheels range from in excess of 105, up to 107, cycles of use),[5] high specific energy (100–130 W·h/kg, or 360–500 kJ/kg),[5][6] and large maximum power output. The energy efficiency (ratio of energy out per energy in) of flywheels, also known as round-trip efficiency, can be as high as 90%. Typical capacities range from 3 kWh to 133 kWh.[2] Rapid charging of a system occurs in less than 15 minutes.[7] The high specific energies often cited with flywheels can be a little misleading as commercial systems built have much lower specific energy, for example 11 W·h/kg, or 40 kJ/kg.[8]
Here m {\displaystyle m} is the integral of the flywheel's mass, and n m {\displaystyle n_{m}} is the rotational speed (number of revolutions per second).
The maximal specific energy of a flywheel rotor is mainly dependent on two factors: the first being the rotor's geometry, and the second being the properties of the material being used. For single-material, isotropic rotors this relationship can be expressed as[9]
where
The highest possible value for the shape factor[10] of a flywheel rotor, is K = 1 {\displaystyle K=1} , which can be achieved only by the theoretical constant-stress disc geometry.[11] A constant-thickness disc geometry has a shape factor of K = 0.606 {\displaystyle K=0.606} , while for a rod of constant thickness the value is K = 0.333 {\displaystyle K=0.333} . A thin cylinder has a shape factor of K = 0.5 {\displaystyle K=0.5} . For most flywheels with a shaft, the shape factor is below or about K = 0.333 {\textstyle K=0.333} . A shaft-less design[12] has a shape factor similar to a constant-thickness disc ( K = 0.6 {\textstyle K=0.6} ), which enables a doubled energy density.
For energy storage, materials with high strength and low density are desirable. For this reason, composite materials are frequently used in advanced flywheels. The strength-to-density ratio of a material can be expressed in Wh/kg (or Nm/kg); values greater than 400 Wh/kg can be achieved by certain composite materials.
Several modern flywheel rotors are made from composite materials. Examples include the carbon-fiber composite flywheel from Beacon Power Corporation[13] and the PowerThru flywheel from Phillips Service Industries.[14] Alternatively, Calnetix utilizes aerospace-grade high-performance steel in their flywheel construction.[15]
For these rotors, the relationship between material properties, geometry and energy density can be expressed by using a weighed-average approach.[16]
One of the primary limits to flywheel design is the tensile strength of the rotor. Generally speaking, the stronger the disc, the faster it may be spun, and the more energy the system can store. (Making the flywheel heavier without a corresponding increase in strength will slow the maximum speed the flywheel can spin without rupturing, hence will not increase the total amount of energy the flywheel can store.)
When the tensile strength of a composite flywheel's outer binding cover is exceeded, the binding cover will fracture, and the wheel will shatter as the outer wheel compression is lost around the entire circumference, releasing all of its stored energy at once; this is commonly referred to as "flywheel explosion" since wheel fragments can reach kinetic energy comparable to that of a bullet. Composite materials that are wound and glued in layers tend to disintegrate quickly, first into small-diameter filaments that entangle and slow each other, and then into red-hot powder; a cast metal flywheel throws off large chunks of high-speed shrapnel.
For a cast metal flywheel, the failure limit is the binding strength of the grain boundaries of the polycrystalline molded metal. Aluminum in particular suffers from fatigue and can develop microfractures from repeated low-energy stretching. Angular forces may cause portions of a metal flywheel to bend outward and begin dragging on the outer containment vessel, or to separate completely and bounce randomly around the interior. The rest of the flywheel is now severely unbalanced, which may lead to rapid bearing failure from vibration, and sudden shock fracturing of large segments of the flywheel.
Traditional flywheel systems require strong containment vessels as a safety precaution, which increases the total mass of the device. The energy release from failure can be dampened with a gelatinous or encapsulated liquid inner housing lining, which will boil and absorb the energy of destruction. Still, many customers of large-scale flywheel energy-storage systems prefer to have them embedded in the ground to halt any material that might escape the containment vessel.
Flywheel energy storage systems using mechanical bearings can lose 20% to 50% of their energy in two hours.[17] Much of the friction responsible for this energy loss results from the flywheel changing orientation due to the rotation of the earth (an effect similar to that shown by a Foucault pendulum). This change in orientation is resisted by the gyroscopic forces exerted by the flywheel's angular momentum, thus exerting a force against the mechanical bearings. This force increases friction. This can be avoided by aligning the flywheel's axis of rotation parallel to that of the earth's axis of rotation.[citation needed]
Conversely, flywheels with magnetic bearings and high vacuum can maintain 97% mechanical efficiency, and 85% round trip efficiency.[18]
When used in vehicles, flywheels also act as gyroscopes, since their angular momentum is typically of a similar order of magnitude as the forces acting on the moving vehicle. This property may be detrimental to the vehicle's handling characteristics while turning or driving on rough ground; driving onto the side of a sloped embankment may cause wheels to partially lift off the ground as the flywheel opposes sideways tilting forces. On the other hand, this property could be utilized to keep the car balanced so as to keep it from rolling over during sharp turns.[19]
When a flywheel is used entirely for its effects on the attitude of a vehicle, rather than for energy storage, it is called a reaction wheel or a control moment gyroscope.
The resistance of angular tilting can be almost completely removed by mounting the flywheel within an appropriately applied set of gimbals, allowing the flywheel to retain its original orientation without affecting the vehicle (see Properties of a gyroscope). This does not avoid the complication of gimbal lock, and so a compromise between the number of gimbals and the angular freedom is needed.
The center axle of the flywheel acts as a single gimbal, and if aligned vertically, allows for the 360 degrees of yaw in a horizontal plane. However, for instance driving up-hill requires a second pitch gimbal, and driving on the side of a sloped embankment requires a third roll gimbal.
Although the flywheel itself may be of a flat ring shape, a free-movement gimbal mounting inside a vehicle requires a spherical volume for the flywheel to freely rotate within. Left to its own, a spinning flywheel in a vehicle would slowly precess following the Earth's rotation, and precess further yet in vehicles that travel long distances over the Earth's curved spherical surface.
A full-motion gimbal has additional problems of how to communicate power into and out of the flywheel, since the flywheel could potentially flip completely over once a day, precessing as the Earth rotates. Full free rotation would require slip rings around each gimbal axis for power conductors, further adding to the design complexity.
To reduce space usage, the gimbal system may be of a limited-movement design, using shock absorbers to cushion sudden rapid motions within a certain number of degrees of out-of-plane angular rotation, and then gradually forcing the flywheel to adopt the vehicle's current orientation. This reduces the gimbal movement space around a ring-shaped flywheel from a full sphere, to a short thickened cylinder, encompassing for example ± 30 degrees of pitch and ± 30 degrees of roll in all directions around the flywheel.
An alternative solution to the problem is to have two joined flywheels spinning synchronously in opposite directions. They would have a total angular momentum of zero and no gyroscopic effect. A problem with this solution is that when the difference between the momentum of each flywheel is anything other than zero the housing of the two flywheels would exhibit torque. Both wheels must be maintained at the same speed to keep the angular velocity at zero. Strictly speaking, the two flywheels would exert a huge torqueing moment at the central point, trying to bend the axle. However, if the axle were sufficiently strong, no gyroscopic forces would have a net effect on the sealed container, so no torque would be noticed.
To further balance the forces and spread out strain, a single large flywheel can be balanced by two half-size flywheels on each side, or the flywheels can be reduced in size to be a series of alternating layers spinning in opposite directions. However this increases housing and bearing complexity.
In the s, flywheel-powered buses, known as gyrobuses, were used in Yverdon (Switzerland) and Ghent (Belgium) and there is ongoing research to make flywheel systems that are smaller, lighter, cheaper and have a greater capacity. It is hoped that flywheel systems can replace conventional chemical batteries for mobile applications, such as for electric vehicles. Proposed flywheel systems would eliminate many of the disadvantages of existing[when?] battery power systems, such as low capacity, long charge times, heavy weight and short usable lifetimes. Flywheels may have been used in the experimental Chrysler Patriot, though that has been disputed.[20]
Flywheels have also been proposed for use in continuously variable transmissions. Punch Powertrain is currently working on such a device.[21]
During the s, Rosen Motors developed a gas turbine powered series hybrid automotive powertrain using a 55,000 rpm flywheel to provide bursts of acceleration which the small gas turbine engine could not provide. The flywheel also stored energy through regenerative braking. The flywheel was composed of a titanium hub with a carbon fiber cylinder and was gimbal-mounted to minimize adverse gyroscopic effects on vehicle handling. The prototype vehicle was successfully road tested in but was never mass-produced.[22]
In , Volvo announced a flywheel system fitted to the rear axle of its S60 sedan. Braking action spins the flywheel at up to 60,000 rpm and stops the front-mounted engine. Flywheel energy is applied via a special transmission to partially or completely power the vehicle. The 20-centimetre (7.9 in), 6-kilogram (13 lb) carbon fiber flywheel spins in a vacuum to eliminate friction. When partnered with a four-cylinder engine, it offers up to a 25 percent reduction in fuel consumption versus a comparably performing turbo six-cylinder, providing an 80 horsepower (60 kW) boost and allowing it to reach 100 kilometres per hour (62 mph) in 5.5 seconds. The company did not announce specific plans to include the technology in its product line.[23]
In July GKN acquired Williams Hybrid Power (WHP) division and intends to supply 500 carbon fiber Gyrodrive electric flywheel systems to urban bus operators over the next two years.[24] As the former developer name implies, these were originally designed for Formula one motor racing applications. In September , Oxford Bus Company announced that it is introducing 14 Gyrodrive hybrid buses by Alexander Dennis on its Brookes Bus operation.[25][26]
Flywheel systems have been used experimentally in small electric locomotives for shunting or switching, e.g. the Sentinel-Oerlikon Gyro Locomotive. Larger electric locomotives, e.g. British Rail Class 70, have sometimes been fitted with flywheel boosters to carry them over gaps in the third rail. Advanced flywheels, such as the 133 kWh pack of the University of Texas at Austin, can take a train from a standing start up to cruising speed.[2]
The Parry People Mover is a railcar which is powered by a flywheel. It was trialled on Sundays for 12 months on the Stourbridge Town Branch Line in the West Midlands, England during and and was intended to be introduced as a full service by the train operator London Midland in December once two units had been ordered. In January , both units are in operation.[27]
FES can be used at the lineside of electrified railways to help regulate the line voltage thus improving the acceleration of unmodified electric trains and the amount of energy recovered back to the line during regenerative braking, thus lowering energy bills.[28] Trials have taken place in London, New York, Lyon and Tokyo,[29] and New York MTA's Long Island Rail Road is now investing $5.2m in a pilot project on LIRR's West Hempstead Branch line.[30] These trials and systems store kinetic energy in rotors consisting of a carbon-glass composite cylinder packed with neodymium-iron-boron powder that forms a permanent magnet. These spin at up to 37,800 rpm, and each 100 kW (130 hp) unit can store 11 megajoules (3.1 kWh) of re-usable energy, approximately enough to accelerate a weight of 200 metric tons (220 short tons; 197 long tons) from zero to 38 km/h (24 mph).[29]
Flywheel power storage systems in production as of had storage capacities comparable to batteries and faster discharge rates. They are mainly used to provide load leveling for large battery systems, such as an uninterruptible power supply for data centers as they save a considerable amount of space compared to battery systems.[31]
Flywheel maintenance in general runs about one-half the cost of traditional battery UPS systems. The only maintenance is a basic annual preventive maintenance routine and replacing the bearings every five to ten years, which takes about four hours.[7] Newer flywheel systems completely levitate the spinning mass using maintenance-free magnetic bearings, thus reducing bearing maintenance and failures.[7]
Costs of a fully installed flywheel UPS (including power conditioning) were (in ) about $330 per kilowatt (for 15 seconds full-load capacity).[32]
A long-standing niche market for flywheel power systems are facilities where circuit breakers and similar devices are tested: even a small household circuit breaker may be rated to interrupt a current of 10,000 or more amperes, and larger units may have interrupting ratings of 100,000 or 1,000,000 amperes. The enormous transient loads produced by deliberately forcing such devices to demonstrate their ability to interrupt simulated short circuits would have unacceptable effects on the local grid if these tests were done directly from building power. Typically such a laboratory will have several large motor–generator sets, which can be spun up to speed over several minutes; then the motor is disconnected before a circuit breaker is tested.
Tokamak fusion experiments need very high currents for brief intervals (mainly to power large electromagnets for a few seconds).
Also the non-tokamak: Nimrod synchrotron at the Rutherford Appleton Laboratory had two 30 ton flywheels.
The Gerald R. Ford-class aircraft carrier will use flywheels to accumulate energy from the ship's power supply, for rapid release into the electromagnetic aircraft launch system. The shipboard power system cannot on its own supply the high power transients necessary to launch aircraft. Each of four rotors will store 121 MJ (34 kWh) at rpm. They can store 122 MJ (34 kWh) in 45 secs and release it in 2–3 seconds.[35] The flywheel energy densities are 28 kJ/kg (8 W·h/kg); including the stators and cases this comes down to 18.1 kJ/kg (5 W·h/kg), excluding the torque frame.[35]
This was a design funded by NASA's Glenn Research Center and intended for component testing in a laboratory environment. It used a carbon fiber rim with a titanium hub designed to spin at 60,000 rpm, mounted on magnetic bearings. Weight was limited to 250 pounds (110 kilograms). Storage was 525 Wh (1.89 MJ) and could be charged or discharged at 1 kW (1.3 hp), leading to a specific energy of 5.31 W⋅h/kg and power density of 10.11 W/kg.[36] The working model shown in the photograph at the top of the page ran at 41,000 rpm on September 2, .[37]
The Montezooma's Revenge roller coaster at Knott's Berry Farm was the first flywheel-launched roller coaster in the world and is the last ride of its kind still operating in the United States. The ride uses a 7.6 tonnes flywheel to accelerate the train to 55 miles per hour (89 km/h) in 4.5 seconds.
The Incredible Hulk roller coaster at Universal's Islands of Adventure features a rapidly accelerating uphill launch as opposed to the typical gravity drop. This is achieved through powerful traction motors that throw the car up the track. To achieve the brief very high current required to accelerate a full coaster train to full speed uphill, the park utilizes several motor-generator sets with large flywheels. Without these stored energy units, the park would have to invest in a new substation or risk browning-out the local energy grid every time the ride launches.
The compensated pulsed alternator (compulsator) is one of the most popular choices of pulsed power supplies for fusion reactors, high-power pulsed lasers, and hypervelocity electromagnetic launchers because of its high energy density and power density.[38] Instead of having a separate flywheel and generator, only the large rotor of the low-inductance alternator stores energy. (See also Homopolar generator.)[39]
Using a continuously variable transmission (CVT), energy is recovered from the drive train during braking and stored in a flywheel. This stored energy is then used during acceleration by altering the ratio of the CVT.[40] In motor sports applications this energy is used to improve acceleration rather than reduce carbon dioxide emissions – although the same technology can be applied to road cars to improve fuel efficiency.[41]
Automobile Club de l'Ouest, the organizer behind the annual 24 Hours of Le Mans event and the Le Mans Series, is currently "studying specific rules for LMP1 which will be equipped with a kinetic energy recovery system."[42]
Williams Hybrid Power, a subsidiary of Williams F1 Racing team,[43] have supplied Porsche and Audi with flywheel based hybrid system for Porsche's 911 GT3 R Hybrid[44] and Audi's R18 e-Tron Quattro.[45] Audi's victory in 24 Hours of Le Mans is the first for a hybrid (diesel-electric) vehicle.[46]
Flywheels are sometimes used as short term spinning reserve for momentary grid frequency regulation and balancing sudden changes between supply and consumption. No carbon emissions, faster response times and ability to buy power at off-peak hours are among the advantages of using flywheels instead of traditional sources of energy like natural gas turbines.[47] Operation is very similar to batteries in the same application, their differences are primarily economic.
Beacon Power opened a 5 MWh (20 MW over 15 mins)[18] flywheel energy storage plant in Stephentown, New York in [48] using 200 flywheels[49] and a similar 20 MW system at Hazle Township, Pennsylvania in .[50]
A 0.5MWh (2 MW for 15 min)[51] flywheel storage facility in Minto, Ontario, Canada opened in .[52] The flywheel system (developed by NRStor) uses 10 spinning steel flywheels on magnetic bearings.[52]
Amber Kinetics, Inc. has an agreement with Pacific Gas and Electric (PG&E) for a 20 MW / 80 MWh flywheel energy storage facility located in Fresno, CA with a four-hour discharge duration.[53]
A 30 MW flywheel grid system started operating in China in .[54]
Flywheels may be used to store energy generated by wind turbines during off-peak periods or during high wind speeds.
In , Beacon Power began testing of their Smart Energy 25 (Gen 4) flywheel energy storage system at a wind farm in Tehachapi, California. The system was part of a wind power and flywheel demonstration project being carried out for the California Energy Commission.[55]
Friction motors used to power many toy cars, trucks, trains, action toys and such, are simple flywheel motors.
In industry, toggle action presses are still popular. The usual arrangement involves a very strong crankshaft and a heavy duty connecting rod which drives the press. Large and heavy flywheels are driven by electric motors but the flywheels turn the crankshaft only when clutches are activated.
Flywheels are not as adversely affected by temperature changes, can operate at a much wider temperature range, and are not subject to many of the common failures of chemical rechargeable batteries.[56] They are also less potentially damaging to the environment, being largely made of inert or benign materials. Another advantage of flywheels is that by a simple measurement of the rotation speed it is possible to know the exact amount of energy stored.
Unlike most batteries which operate only for a finite period (for example roughly 10[57] years in the case of lithium iron phosphate batteries), a flywheel potentially has an indefinite working lifespan. Flywheels built as part of James Watt steam engines have been continuously working for more than two hundred years.[58] Working examples of ancient flywheels used mainly in milling and pottery can be found in many locations in Africa, Asia, and Europe.[59][60]
Most modern flywheels are typically sealed devices that need minimal maintenance throughout their service lives. Magnetic bearing flywheels in vacuum enclosures, such as the NASA model depicted above, do not need any bearing maintenance and are therefore superior to batteries both in terms of total lifetime and energy storage capacity, since their effective service lifespan is still unknown. Flywheel systems with mechanical bearings will have limited lifespans due to wear.
High performance flywheels can explode, injuring bystanders with high-speed fragments.[61] Flywheels can be installed below-ground to reduce this risk. While batteries can catch fire and release toxins, there is generally time for bystanders to flee and escape injury.
The physical arrangement of batteries can be designed to match a wide variety of configurations, whereas a flywheel at a minimum must occupy a certain area and volume, because the energy it stores is proportional to its rotational inertia and to the square of its rotational speed. As a flywheel gets smaller, its mass also decreases, so the speed must increase, and so the stress on the materials increases. Where dimensions are a constraint, (e.g. under the chassis of a train), a flywheel may not be a viable solution.[citation needed]
Anker Solix F: The capacity and capabilities of portable power stations continue to evolve, with the largest models being capable of home backup and running high-power appliances. The Anker Solix F is one such portable power station, with plenty of juice and the ability to run just about anything in your home, making it more similar to whole-home backup systems like the EcoFlow Delta Pro Ultra and Tesla Powerwall. It's also worth noting that the F has a successor -- the F Plus, which now takes our top spot for extra-large power stations and home backup. There are a few things I am particularly fond of with the F. For starters, you can add up to six expansion batteries for a total of 26.9Wh capacity. The Solix F boasts a nominal power output of 6,000 watts and the capacity for 120/240 split-phase output in the same unit. It also has both NEMA 14-50 and L14-30 receptacles to directly charge things like your EV, RV or large appliances. You could also choose to pull double duty, run two of these units and max out the expansion batteries for a total of 53.8kWh capacity. Doing so would double your power output to 12,000 watts. It's no slouch in testing either, with a reasonable 79% usable measured capacity and the ability to charge to full in under 3 hours. Bluetooth and Wi-Fi connectivity allow for monitoring energy use through the app, which is a nice touch. It comes with a generous five-year warranty, which is notably longer than the other extra-large portable power station on this list, the Fossibot F Pro.
Anker 555 PowerHouse (1,024Wh) (Update: Currently unavailable): An increasing number of portable power stations are shipping with LifePO4 batteries, and I love that. The 555 is slower to charge than most of its competitors but sports a 94% usable capacity and an attractive price versus the number of watt-hours; the better to power those six AC outlets.
Anker Solix C (1,056Wh): Another good option from Anker. It tested well in our lab and comes with a built-in LED light which is a nice touch for emergencies. Anker currently has it at almost half off and supports a five-year warranty, making it a great option when it's on sale.
Anker Solix C800 Plus (768Wh): So, I kind of like this guy. It only really performed average in our standard tests for power stations, but it has a feature that is quite interesting, if not a bit gimmicky. In a storage compartment on top of the unit, you will find a telescoping pole that can be mounted onto the power station to support one of two rechargeable camping lights. You're not getting a ton of light out of these things, but, some light is infinitely better than no light. Plus you get the hands-free mode with the telescoping pole mount -- maybe that is what the 'plus' is for?
Anker Solix F (1,229Wh): This unit was previously known as the PowerHouse 757 from Anker, and was also CNET's previous pick for "best portable power station for backup." Its UPS mode was one of the earlier units to boast "less than 20ms" switchover time in the event of a power outage. It's also currently $500 off on Anker's site.
Anker Solix F (2,048Wh): Previously known as the Anker PowerHouse 767 and previous winner of "best large portable power station" here on CNET. This model has lots to offer by way of features and options -- pretty much anything other than wireless charging. It also performed well on our usable capacity and charge time tests. It's now been supplanted by the Solix F Plus.
BioLite BaseCharge 600+ (622Wh): BioLite has released upgraded versions of its BaseCharge 600 and models. There's a little give and take here -- the upgraded units charge a bit faster, but have a little less usable capacity. Still, just an 'OK' pick.
BioLite BaseCharge + (1,521Wh): Having tested both the 600 and models of the upgraded BioLite BaseCharge+ line, I can tell you that this company is consistent in its product manufacturing. The BaseCharge + is about 2.5 times the capacity of the 600+. That 2.5 modifier carries across the board fairly accurately from price to capacity, charge times and everything else. If you like the 600+, but you wish you had two and a half of it, save yourself the effort and just buy the +.
Bluetti AC180 (1,152Wh): This unit tested well enough, scoring 88% usable capacity and charging via AC outlet at 13.88 watt-hours per minute. But unlike many of the other Bluetti units that use the same physical format, this unit does not support capacity expansion via external batteries.
Bluetti AC180T (1,433Wh): I like this unit, but I'm just as, if not more, excited about its energy platform: the SwapSolar Ecosystem. Not unlike some of the Runhood units we've tested, this ecosystem uses interchangeable batteries that'll power multiple products from the brand. We have also tested an electric cooler on the same ecosystem. As far as large power stations, the performance metrics were great with this unit, just not quite enough to capture a title.
Bluetti AC2A (204.8Wh): A great option if you don't need a ton of capacity but do need options beyond just USB connectivity. This unit is in the capacity ballpark of a very large power bank and priced similarly. Plus, it's currently on sale through the manufacturer for $149.
Bluetti AC200P (2,000Wh): This is one of Bluetti's earlier large portable power stations and a previous winner for "best large portable power station." It's $400 off on Bluetti's site. It still offers plenty of power and options, but is likely nearing the end of its product cycle lifespan (hence the discount).
Bluetti AC 200 Max (2,048 Wh) (Update: Out of stock): The AC200 Max once held our title for the best value portable power station, giving you a 2,048 watt-hours of capacity (expandable to 8,192Wh), 2,200-watt output (4,800-watt surge) and 900 watts of solar charging power (1,400-watt solar plus AC). That's nothing to laugh at in this price range. Most offerings with similar specs sit closer to $2,000 and often are missing the expandability aspect. The AC200 Max is comparable in form to Bluetti's larger format AC300 and AC500 units.
Bluetti AC240 (1,536Wh): I've tested a dozen or more Bluetti power stations at this point. The AC240 is good, as are most of Bluetti's offerings, but I wasn't particularly blown away and didn't find anything new to be excited about here. It performed slightly below many other Bluetti units on the usable capacity test but has many of the expandability options I admire, and UPS switchover times are coming down across the board -- 15ms on this unit, although I'll be happier when sub-12ms is the standard.
Bluetti EB3A (268Wh): If you're interested in something small to work for your personal charging needs but those pocket-sized battery packs just don't cut it, this could be your option. As a previous CNET best value winner, the EB3A has what you need to keep rocking for a couple of days.
Bluetti EB55 (537Wh): We've liked almost every unit from Bluetti, and three of them took previous titles in this best list, but this unit was overshadowed by its siblings. Offerings that are just as good or better at better prices keep the EB55 out of the winner's circle.
For more information, please visit energy storage sports equipment.
BougeRV Fort (1,120Wh): I'm a fan of BougeRV's approach to camping and outdoor products in this space. It's worth checking out, especially if you're looking for more flexibility in areas like solar panels or DIY options. The Fort did well in our tests but didn't stand out enough to capture any titles.
BougeRV Flash 300 (286Wh) (Update: Out of stock): Another one bites the dust. Once our top pick for the best small portable power station, it came with 600 watts of power, supported wireless charging and could charge to 100% in 45 minutes.
EcoFlow River Max (576Wh) (Update: Currently unavailable): Blazing fast charging and a low-cost per watt-hour make this a reasonable pick, although this unit did test lowest in measured versus expected capacity, putting it at 425 usable watt-hours. Where'd those extra 151 watt-hours go?
Ecoflow River 2 Pro: A previous title holder for "best budget portable power station," this is still a great pick for anyone looking for affordable power options. It charges fully in just over 1 hour and accesses a respectable 82.6% of the battery's 768Wh stated capacity.
EcoFlow Delta 2 (1,024Wh): The EcoFlow Delta 2 is similar to the Anker 555 PowerHouse across the board -- features, pricing and so on. The main differences you can see from our tests are the usable capacity percentages: Anker with 94% versus EcoFlow with about 70% and charging rates. Both are rated at 1,024Wh. The EcoFlow Delta 2 charged to full in only 86 minutes, 275 minutes faster than the Anker model. Another point for EF is that it can wire in a secondary battery module, taking the capacity from 1,024Wh to 2,048Wh. Expect to pay an additional $300 for that battery expansion.
EcoFlow Delta 2 Max (2,048Wh): Another example of a great product that didn't capture any of our titles. The Delta 2 Max performed well in all of our tests, and with the ability to expand to 6.144kWh, you're really walking the line between a portable power station and a whole-home energy solution.
EcoFlow Delta Mini (882Wh) (Update: Out of stock): When we tested it, this model hit a sweet spot of basic functionality, capacity and price, earning it a spot on our list as the best portable power station for camping. It also supported charging with solar panels. Unfortunately, it's no longer available for sale.
EcoFlow Delta Pro (3,600Wh): The EcoFlow Delta Pro is one of the largest portable power stations on our list at 3.6kWh (expandable up to 25kWh), and also happens to be one of the fastest charging. Lots of power and plenty of charge options to keep that power rolling.
Jackery Explorer 240 (240Wh): We've been fans of all the Jackery units we've ever tested in the past, and that doesn't change here. Just missing the best small power station title, this unit still boasts the second-best capacity rating of all the ones we tested. It was a little slow to charge but is offered at a great price.
Jackery Explorer 300 Plus (288Wh): Another nice entry into the platform, the 300 Plus offers a solid power option in small form. Not a ton of frills, but it does what you expect it to do.
Jackery Explorer 700 Plus (680.96Wh): If you need more power output than the 300 Plus (300-watt/600-watt) then the 1,000-watt (2,000-watt surge) of the 700 Plus may be what you're looking for. It will charge via AC in about 1 hour and 30 minutes, and it has one of Jackery's higher usable capacity percentages at 88%.
Jackery Explorer Pro (1,002Wh): The Pro falls into our large portable power station, which begins at 1,000Wh (this Jackery weighs in at 1,002Wh; the same as its big brother, the Pro). I like the more than the for a few reasons, so the never had a shot at taking the "large" category. But it still has good performance, nice features and amazing charge times.
Jackery Explorer Pro (1,512Wh) (Update: Out of stock): With this Jackery you'll get a dependable machine that performs well. In our usable capacity tests it came in at 90.4% and charges quickly: 0 to 100% in 2 hours, with AC-only charging. Toss in a couple of solar panels and you can drop that time down quite a bit.
Jackery Explorer Pro (2,160Wh): This was a previous title-holder of the fastest charging portable power station. The Jackery units overall are great and dependable. If you're looking for a model (really, an entire lineup) that will recharge fast with multiple, even combined options, Jackery is a no-brainer.
Jackery Explorer Pro (3,024Wh): Another beast of a unit and a great offering from Jackery. If you're already a Jackery fan but need more battery capacity, this is an easy win for you. Otherwise, recent improvements include wheels, telescoping handling and that round RV plug we've been waiting for.
Oupes 600W (595Wh): Not a bad little unit. I love that it has the LifePO4 battery. It performed about average (maybe a hair under par) and I feel like it could be cheaper. The name can be hard to pronounce. "Oops" is our best guess.
Oupes Mega 3 (3,072Wh): Another great offering from Oupes with stellar performance. The numbers from our test lab slightly favored the Mega 3 over the Mega 5. We also tested the B2 expansion battery with this unit and further improved the numbers for usable capacity. The Mega line continues to impress with both performance and value.
Oupes Mega 5 (5,040Wh): Previous titleholder for "best extra-large power station," the Mega 5 is a beast. It maxes out at 5,040Wh, with a single expansion battery option, the B5, bringing the total to 10.08kWh. At 4,000-watt output with a 7,000-watt surge, you'll be able to power pretty much anything you want (as long as it uses either a standard 120-volt plug or the round RV type). It also has a large solar charging capacity (4,050-watt) and in our tests proved to be the second fastest charging unit, going from 0 to 100% in 214 minutes, or 3 hours and 54 minutes. That ends up being 23.55-watt hours per minute charged, which is the second-highest rate we've recorded.
4Patriots Power Sidekick (299Wh): The specs on this small unit are ok - 87% usable capacity is about the norm and it charges at about one watt-hour per minute. That's a bit slow, but not unheard-of for these small units. It does come with a small, 40-watt solar panel, but I think it is still overpriced at $500.
4Patriots Power Generator X (.8Wh): While I do think this is a better engineered product than the smaller Sidekick offering from 4Patriots, I still feel like their product lines are overpriced. Again, they do come with small solar panels, but that is hardly worth double the sticker price in many instances. Usable capacity is still in line at 87% but this size unit only charging at 7Wh/min is a bit underpowered in my opinion.
Ampace Andes 600 Pro (584Wh): This is an OK unit. It sits right around the industry standard for usable capacity. We did find that the charging moved much slower than the marketing materials claim: 90 minutes to 80% charge vs. the advertised 60 minutes.
Apace Andes (Wh): There are a couple of notable improvements between the smaller Pro unit I previously tested and this larger Wh unit. For one - weight. The spec capacity is almost three times larger, and normally capacity and weight are proportional, but despite the larger capacity, the weighs only a little over double the smaller unit. This thing charges quickly too. It doesn't quite hold up to the company's 55-minute charge time claim, but one hour and sixteen minutes isn't bad!
Bailibatt 300W (257Wh): Another small, affordable unit. The Bailibatt comes in at 84% usable capacity, which is good. It takes 11 hours to charge, which is.... not as good. If you have specific limited charging needs and plenty of time to recharge, the price tag makes it worth considering.
BigBlue Cellpowa 500 (537.6Wh): This is a better-than-average performing unit at better-than-average pricing, but there's nothing outstanding about it.
Dabbsson DBS (2,300Wh): I love that it's a modular format, expandable up to 8.33kWh. The 87% usable capacity is good and charges relatively quickly. It charges at over 18 watt-hours per minute, for a total of 122 minutes to charge the entire 2,300Wh.
Dakota Lithium PS (2,060.8Wh) Update: Out of stock: The Dakota Lithium PS is the fastest-charging portable power station on our list. Now, looking at our test data, that doesn't mean that it took less time to charge than any other unit, but, in using our residential AC charging method, it instead indicates the unit that charges the most watt-hours per minute. This method allows the large-capacity units to compete in this category with much smaller units that naturally charge much faster. It showed a respectable 90.72% in usable capacity while also giving us our fastest charge metric to date. We showed a charge rate of 26.76 watt-hours per minute, just edging out the Oupes Mega 3 by about 0.3 Wh/m. Also notable is the unit's 10ms switchover time. Charging from 0% to 100% in 77 minutes, the PS sees 26.76 watts-per-minute charging from a standard 120-volt, 20-amp residential outlet. A close second was the Oupes Mega 3 at 26.48 watts per minute, followed by the VTOMAN Flashspeed in a more distant third at 24.9 watts per minute. Different units make up the bulk of the next-best contenders, from companies like Oupes, VTOMAN, UGreen and Goal Zero. If charging to recover your total capacity is a major concern for you, these are the units to look at. In addition, they all offer simultaneous charging from other inputs like solar or other DC inputs if you need to up your recharge game.
DaranEner NEO (2,073.6Wh): This unit didn't win any categories, but it did perform in the top tier for our charge tests and came in about average for our usable battery capacity tests. This sturdy unit has plenty of features and one of the lowest prices per watt-hour.
Deeno GT S (1,036Wh): We previously tested the Deeno GT X, and the S is a big step up. It has the same capacity and same pricing, but with nearly 20% more usable capacity than the previous model and it charges nearly five times faster.
Deeno X (1,036Wh): The X did not fare well in our tests. It came through with one of the lowest usable capacity scores we've collected so far at 69.88%, meaning you see about 724Wh out of the stated 1,036Wh. For the price, there are better options.
DJI Power (1,024Wh): I want to note first off that I do not have a drone that's compatible with the SDC super-fast charge function for select DJI drones. I think that's likely to be one of the best selling points for this particular unit. I'm also happy that there are two 140-watt USB C ports. Outside of those features, there isn't much else to talk about. It does charge fast but it came in low on our usable capacity test.
Duracell Power 500 (515Wh): This is the first Duracell unit I've tested, but not the first battery brand to put out a portable power station (see Energizer at the top of this list). So far, the results are similar. Test results come back with under-average performance and questionable prices.
Duracell M250 (219Wh): Overall, this smaller unit is proportionally comparable to the larger Power 500 Duracell model. The M250 came in at 75% usable capacity, just a couple of points higher than the Power 500. You're getting approximately half the capacity for half the price. Charging is also in line, taking around the same time (4+ hours) to charge half the capacity (at half the input power). I like the cylindrical shape -- I'm guessing Duracell wants it to look like that familiar battery profile -- and that the lid opens up to allow for power cable storage within the unit.
EBL MP (999Wh): This is not a bad little unit. Doesn't charge too quickly, but overall usable battery capacity is good. It has a wireless charge pad and lots of options for inputs and outputs. It isn't something I'd run out to buy for myself, but if you can catch it on sale, it could be a low-cost way into a smaller power station.
Encalife UAF550 (595Wh): Of the three Encalife chargers, this has the largest usable capacity percentage at 87% but the slowest charging at 1.98-watt hours per minute.
Encalife UAF (992Wh): Industry standard usable capacity here at about 84%, but a bigger drop in the charge capabilities at 3.35-watt hours per minute from its larger sibling.
Encalife YUE (2,048Wh): A bit of variation in our model hierarchy groupings with Encalife. As you might expect, charging capabilities do increase with larger units. The YUE being the largest of the three charges relatively quickly, at about 11.13 watt-hours per minute. In this series, the usable capacities trend in the other direction, with this unit showing 73% usable capacity.
Enernova ETA 288 (288Wh): This is another example of a hierarchy of models where the smaller units underperform, but larger models improve. This unit took about 3 hours and 40 minutes to charge, but it reached about 81% usable capacity.
Enernova ETA Pro (1,050Wh): Moving up a notch, this one has 83% usable capacity and charges 1kW in about 1 hour and 30 minutes. It's a better showing and about 10 cents cheaper per watt-hour than its smaller sibling.
Enernova ETA Ultra (2,150Wh): This is the best of the three, sporting Wh, 87% usable capacity and it charges in under 2 hours.
Energizer PPS700 (626Wh) (Update: Currently Unavailable): OK performance and features overall, but one of the lowest-tested capacities, making the usable capacity closer to 477Wh.
Etaker M (Wh): While the M didn't win any of our categories, I do like the platform. More manufacturers are offering increased capabilities like ones you'll see here: semi-solid state battery engineering, modular platform to increase capacity, ability to expand to 240VAC, direct EV charging and a few more use cases that are more niche. It's a reasonable grab, especially if you're looking for something more sturdy like cold weather charging -- all with a usable capacity percentage of 91 -- above average for the units we normally test.
Fanttik Evo 300 (299Wh): This is a solid pick in the small power station category, and this unit has my favorite display: It's extra large and easy to read. We did see average performances on our charging and capacity tests.
Generac GB (1,086Wh): Generac has been a household name in the power landscape (especially generators) for dozens of years in the US. It isn't that surprising to see it here in the portable battery space. I wasn't blown away by this unit. It tested quite average (if not a smidge below on charge speed) but overall, still does what you need: to transport power.
Geneverse HomePower One (1,002Wh): This unit was the second slowest overall to charge, but did well on its usable capacity rating at 91%. Its display is small but offers all the standard input and output features you'd want.
Geneverse HomePower One Pro (1,210Wh): This is the grownup version of the Geneverse HomePower One. The feature specs are about the same, but at $500 more, you're only getting about 200 extra watt-hours. In addition, the standard One model comes in at 91% usable capacity versus the Pro model's 73%. That gives you 912.6 usable watt-hours with the standard and only 886.7Wh on the Pro. The Pro charged in almost a quarter of the time it took the standard version.
Goal Zero Yeti 200X (187Wh):: The Goal Zero products are solidly made, but we got the lowest score in our "usable capacity" tests from this unit. It's about 65% compared to the industry-accepted norm of 85%. There are better products in the small portable power station category.
Goal Zero Yeti 700 (677Wh): Overall this unit tested OK; nothing outstanding. One of the more interesting aspects of the design is the use of plastic protective 'flaps' covering the in/output ports. The use case here seems to be for a more rugged durability for camping or otherwise roughing it where you might have an increased exposure to mud, dirt, dust or water. Outside of those environments, the flaps can be bothersome.
Goal Zero Yeti Pro (3,993.6Wh): Runner-up for our best extra-large power station title, the Yeti Pro is a tank (which, by the way, is the name of the expansion battery: "Tank Pro "). You get tons of input and output options, and it is expandable to 20kW capacity. We were able to charge this via standard AC outlet in 2 hours and 49 minutes, giving us our third-fastest charging rate so far at 23.63 Wh/min charged. It also offers 3,000-watt solar input. If you're looking into home backup, also check out the Haven10 transfer switch accessory to bring your home online.
GoSun PowerBank (1,100Wh): I wanted to like this unit more, partially because of GoSun's extended offerings of solar-friendly devices. As far as capacity goes, this runs in the middle of the pack, but man is it slow to charge. It took nearly 12 hours -- over six times as long as our largest power station (Jackery Explorer Pro) -- which offers nearly twice the capacity. At $1,199, I'd like to see a faster charging option and more outputs or at least wireless charging.
Lion Energy Safari (1,612Wh): I tested two units, the first I've had my hands on, from Lion Energy. Two immediate observations on the superficial side: First, I like the physical packaging of these units. It reminds me of Oupes, which I also like. Second, these things sound like the names of energy drinks. Otherwise, I wasn't impressed. This larger unit also had a bad connection to its display screen. In general, the color screen looks better than most other options. In my case, I had several lines plaguing each view.
Lion Energy Summit (665Wh): The TL;DR is above, with the larger unit, but for additional context, units tested about average in terms of usable capacity. My main performance complaint is the charge speed. Both units are a bit low for their capacity category. I would be happier if the Summit charged at the Safari rate, and the Safari was retooled for nearly double its current speed.
Litheli PowerHUB B600 (562Wh): This one can be slow to charge, but otherwise, there's a lot to like here. It has good usable capacity at a decent price, since it's currently marked at about 40% off. Litheli is also offering a battery platform (U-Battery) with this unit. Two smaller batteries plug into the main unit that you can then use with a variety of other tools. Check out our upcoming coverage on handheld vacuums to see Litheli's performance there.
Litheli PowerHUB Eclair (1,069Wh): Another unit from Litheli offering the U-Battery platform, but it's closer to double the capacity of the last unit we tested. The capacity score was low, but the charge speed has vastly improved from the B600. Worth a look if you're interested in the U-Battery platform.
Mango Power E (3,530Wh): I mentioned this unit earlier as the runner-up in the fastest charging category. This thing is loaded with features, even allowing you to provide 240-volt service by linking a second unit. There are also battery expansions for the Mango Power E. The one downside is the price tag, as this unit also comes through as the most expensive portable power station with a list price of $4,250.
Milwaukee M18 Carry-On Power Supply: It isn't a great portable power station. You're probably considering buying it because you're already on the platform. If that's the case, I say go for it. It will work with all batteries on the M18 platform, so you have some control over how much juice you want to carry around. This will put a fair amount of convenience into situations for people who are Milwaukee tool users and find themselves in energy-shy situations.
Monster Power Grid 300 (296Wh): The Power Grid 300 can be slow to charge but did test at over 90% usable capacity. It has all the bells and whistles you'd expect at this level at a price that's a tad high.
Oscal PowerMax 700 (666Wh): Another unit that didn't perform particularly well in our tests, but does boast a ton of features, including a "non-stop continuous power supply mode."
Oukitel BP (2,048Wh): This is the first unit we've tested from Oukitel (along with its expansion battery -- we will be publishing more on expansion batteries soon) and we were pleasantly surprised. To begin, the BP scored an impressive 90% on usable battery capacity and also scored well in our charge tests, taking less than 2 hours to charge all Wh of capacity. Oukitel is also leaning into the modular and expandable approach, allowing you to add up to seven additional units for a total of just over 16kWh of power. The BP also boasts sub 10ms switchover time as its UPS feature.
Pecron E (1,536Wh): I will say that I'm a bit torn with this unit. I tested the ELFP model first and was impressed with the results. The ELFP did not fare as well. Specifically, the usable capacity came in especially low at 68% compared to the E at 94%. Other than that, the charging is beefier, charging at about twice the rate as the other unit. And you still get perks like wireless charging and expandability from Wh up to Wh.
Pecron ELFP (1,920Wh): I discussed this unit briefly earlier as the runner-up to the Delta Mini in the "best portable power station for camping" race. It has more options than the Mini and is suitably priced. I'm also a fan of any of the companies that adopt the modular approach with the capability to expand capacity with external batteries, like Pecron has done. You can also pick up a rolling caddy for the unit if you're on the go.
Phyleko ENFS (1,024Wh) (Update: Currently Unavailable): I've seen this body style before in the GoSun ; it feels super sturdy and I do like the larger colorful display. Otherwise, this unit landed just under average in our tests.
Power Cache 300 (293.76Wh): We tested a trio of power stations from Power Cache. The 300 model did well as far as usable capacity goes (91%) but took over 7 hours to charge. Another upside is that it's affordable, with a $200 retail tag.
Power Cache 600 (642.6Wh): Costing $250 more than its smaller sibling, this middle child showed the least impressive performance of all three models, coming in at only 72% usable capacity and taking over 8 hours to charge to 100%. If price is your main concern, it's an affordable option.
Power Cache (1,075Wh): The largest of the three units, this one performed moderately, coming in at 82% usable capacity and taking about the same amount of time to charge as the small 300 model, which is just over 7 hours. That does mean it charges over four times faster than the small unit, but 7 hours is still 7 hours. Selling at $470.
Renogy Phoenix 200 (189Wh) (Update: Currently Unavailable): Slower to charge, but it has 96% usable battery capacity paired with the lowest price of any unit we've tested. This a great option for smaller use cases or for people generally interested in checking out portable power stations at a reasonable price.
Renogy (998.4Wh) (Update: Currently Unavailable): This is another decent performer. It charges fast enough for its relative capacity category, but only offered us about 80% usable capacity. Normally, I wouldn't be too bothered, but the smaller Renogy unit we tested clocked in at 96% usable capacity, so I was hoping for more.
Rockpals 300W (Update: Currently Unavailable): This unit also came in under the line in usable capacity. Given the industry standard of 85%, Rockpals' 78% is lacking. In terms of charge speed, this unit is one of the faster small portable power stations. It has decent features and kind of looks like a handheld radio.
Rocksolar Nomad RS650 (444Wh): Until the company updates this unit, there are likely better options for almost anything you're looking to do. It has a high price, low usable capacity, slow charge time and is low on features and options, but it does work.
Runhood Rallye 600 (648Wh): There are a couple of these types of units on the market now, and I've been waiting for their arrival. This Runhood unit is the first modular-style portable power station I've been able to get my hands on, and I love what it means for the industry. Performance-wise, this model was about average, but it could offer you more flexibility and convenience than many other units. The batteries are swappable, so you can pick up extras, in addition to standalone AC and USB modules that can use those extra batteries without being plugged into the main power station unit. This could be a game-changer for trips where every member of the family is off in a different area draining some electronic device. I look forward to adding a "best modular power station" category soon.
Runhood Rallye (648Wh) (Update: Currently Unavailable): The capacity is the same for this unit as it is for its younger sibling, the 600 (listed above). The 600 and designations refer to the constant power output in watts, with each unit's peak power doubling that constant power number. You do get an extra AC outlet but the increase in power output is the main difference. Likely worth it if you're into the modular design, but need more power than the 600 has to offer.
Segway Cube (Wh): I am always excited to check out new entries into the portable power station category, especially from companies that are already in similar spaces. The Segway Cubes are a solid first product, leaning into modularity and avoiding an attempt to match all the latest bells and whistles. The units show an average usable capacity percentage, the same for charge speed.
Segway Cube (Wh): The Segway Cubes are offered in retail capacities of either 1 or 2kW. The platform is expandable to 5kW. The interesting choice is Segway's decision to offer both the 1kW and 2kW retail options, when both options are otherwise identical and identically expandable. It seems that either retail option contains an identical inverter and 1kW battery. The Cube offers an additional battery expansion that goes between the two previously mentioned units. At that point, you can add a total of four battery expansions to max out the platform at 5kW.
Togo Power Advance 346 (346Wh): This unit held the title for best small portable power station for about two years on this list; solid performance, great features and an attractive price tag.
Ugreen Power Roam 600 (680Wh): This unit didn't do great in our tests, but it has a reasonable price. It charges quickly, but that has more to do with the smaller capacity than an elevated charging capability.
Ugreen PowerRoam (2,048Wh): I was happy to see that this model did better than the previous smaller model we tested. 83% on usable capacity and it charged in the same amount of time as the smaller unit, about 1.5 hours for each one, which means the was charging at about four times the rate. This one also has wheels and a telescoping handle for ease of movement.
VTOMAN Flashspeed (828Wh): This is the second VTOMAN Flashspeed I've tested and the smaller of the two. This smaller unit didn't fare as well as its big brother in our tests. While it's a capable machine, I struggle to find a scenario where the minimal cost difference between the two would keep me from buying the over the .
VTOMAN Flashspeed (1,548Wh): This unit did about average on our usable capacity test, but, charging from 0 to 100% in 64 minutes, the FlashSpeed sees 24.19 watts-per-minute charging from a standard 120-volt, 20-amp residential outlet, which is one of the fastest charge rates we've seen here in the test labs. If charge speed and time are primary concerns for you, it's almost impossible to do better than the Flashspeed .
Yoshino B330 SST (241Wh): This is a cute little power station. It has a USB-C 100-watt port, a couple of AC outlets and is pretty light for what it offers. It comes in just above average for usable capacity but does have a slow charge rate. If you're looking for a little power in a compact package, it's worth considering.
Yoshino BSST (2,611Wh): This unit tested fairly well in our lab. 87% usable capacity, blazing-fast charge speeds and a decent feature set. It's an option worth considering, if you can find it on sale.
Zendure SuperBase Pro (2,096Wh) (Update: Out of stock): The first unit we tested with the Li-NMC battery composition. This unit also just missed the best large portable power station title. It has a weight-to-capacity ratio, likely thanks to the NMC composition, and boasts our highest solar charging capacity to date at 2,400 watts. Its telescoping handle and wheels make it easier to manage, but the form makes it better for navigating paved walkways than "off-road" terrain.
70mai Hiker 400 (378Wh) (Update: Out of stock): This unit didn't fare too well in our tests, coming in at about 75% usable capacity (versus the industry standard of 85%) and taking about 4 hours and 30 minutes to charge its 378Wh.
70mai Tera (.9Wh): The larger of the two 70mai units tested better, hitting the industry standard for usable capacity and taking about 20 minutes less to charge nearly three times the capacity of the smaller model.
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