Power bank specifications are unusually honest about the wrong things. The capacity printed in giant numbers on the front is real, and you will never get it. The wattage is real, and it often is not available on the port you are using. Learning to read these claims is worth more than any ranking, because it tells you which of two similar looking packs will actually do the job.
Quick answer
Expect about 70 to 75 percent of the printed capacity to reach your device, because converting voltage costs energy as heat. Check whether the headline wattage is per port or shared across all of them. Ignore the number of charge cycles a brand promises and look instead at what the pack does when two things are plugged in. And read the watt hour figure on the case, because that is the number airlines enforce. Our full picks are in the power bank buying guide.
This page is about how these products are measured and how the claims are worded, so you can judge a spec sheet yourself instead of taking a verdict on faith.
Capacity: why you never get what it says
The cells inside a power bank sit at roughly 3.7 volts. Your phone wants 5 volts or more at the connector. Converting between the two wastes energy as heat, and there is no design that avoids it, only designs that waste less.
In practice that means a 10,000mAh pack delivers somewhere around 7,000mAh to a device. A brand claiming two and a half full phone charges from 10,000mAh is quoting a small phone and a perfect conversion. Efficiency also gets worse when the pack is hot, when it is charging at maximum speed, and as the cells age.
The useful comparison between two packs is therefore not the mAh number, which is set by the cells, but whether the manufacturer publishes a realistic output figure at all. A company printing an honest usable capacity is telling you something about the rest of its claims.
| The claim | What it means | What to check |
|---|---|---|
| 20,000mAh | Cell capacity at 3.7 volts, before conversion losses. | Assume roughly 14,000mAh reaches your devices. |
| 100W output | The maximum the pack can deliver, often from one specific port and often only when nothing else is connected. | Find the per port table in the manual, not the headline on the box. |
| Fast recharge | How quickly the pack refills itself, which is a different number from its output. | Anything under 30W on a large pack means an overnight refill. |
| 74 Wh | The energy figure, printed small near the certification marks. | This is the number that decides whether it flies. Under 100 Wh is unrestricted. |
| Water resistant | Usually means splash resistant, and frequently covers the case but not the ports. | Look for an actual IP rating. No rating means the phrase is marketing. |
How wattage gets shared
This is the specification that catches people out most often. A pack advertising 100W frequently means 100W from one USB C port with nothing else attached. Plug a phone into the second port and the first may drop to 65W or even 45W, because the total the pack can deliver is fixed and gets divided.
That matters if you planned to charge a laptop and a phone at once on a train. Manufacturers do publish the split, usually as a small table in the manual or a footnote on the product page, listing combinations such as one port at 100W, two ports at 65W and 30W. If a company does not publish that table, assume the worst case.
Note
The cable is part of the circuit. A 100W capable pack pushing through a cheap cable rated for 60W charges at 60W, and many cables sold as charging cables carry no rating at all. For anything above 60W the cable needs to be rated for it, which usually means one with an internal chip.
What durability claims are worth
Drop resistance, water resistance and cycle life are the three claims most often made and least often specified. Drop resistance without a stated height and surface is meaningless. Water resistance without an IP rating is a marketing word. Cycle life, usually quoted as hundreds or thousands of cycles, is measured under laboratory conditions at moderate temperature and says nothing about a pack that lives in a hot car.
The claim that does carry weight is certification. Marks such as UL, CE and UKCA, and the presence of a full watt hour figure and a model number on the case, mean the product has been through a compliance process. A pack with no markings at all, bought from an unfamiliar seller, is the category where fires start.
Warning
Check recall notices for the pack you already own, not just the one you are considering. Major brands have withdrawn batches of power banks in recent years and publish serial number checkers when they do. The check takes two minutes and is the single highest value thing in this article for anyone with a drawer full of old batteries.
Reading a review critically
Most published power bank rankings measure capacity by discharging into a load at a fixed rate and timing it. That is a good test, and it is also the easiest one to pass, because capacity is the specification manufacturers get right. The things that separate a good pack from a mediocre one, how it behaves when hot, how it splits power between ports, how it ages over two years, are much harder to test and are correspondingly rare in reviews.
So when a roundup says a pack was tested, it is worth asking what was tested. A measured capacity figure is real evidence. A verdict on durability after a fortnight is not, and any claim about how a battery will perform in three years is a prediction rather than a result.
Frequently asked questions
Why does my 10,000mAh power bank only charge my phone once and a half?
Because roughly a quarter to a third of the rated capacity is lost converting the cell voltage to what your phone accepts, and a modern phone battery is larger than it used to be. Expect around 7,000mAh of usable output from a 10,000mAh pack, which on a large phone is one full charge plus a useful top up.
Does a 100W power bank really deliver 100W?
From one port, with nothing else connected, usually yes. Connect a second device and the total gets divided, often leaving 65W on the main port. Manufacturers publish the split in the manual, and if they do not, assume the lower figure.
What do watt hours mean on a power bank?
Watt hours describe stored energy rather than charge, and that is the unit airlines and regulators use. Multiply the mAh figure by 3.7 and divide by 1,000 to get it. Compliant packs print the figure on the case, and under 100 watt hours travels without special permission.
Are expensive power banks actually better?
Often, but not in capacity. The cells are broadly similar across the market. What money buys is better power management, an honest and properly documented port split, faster recharging, real certification and a company that will handle a recall properly. Those are the parts you cannot see on the box.
The bottom line
Read three numbers before you buy: the watt hour figure for flying, the per port wattage for how it will actually be used, and any published usable capacity as a signal of how straight the manufacturer is being. For specific recommendations by use case, see our guide to the best power banks and portable chargers.
