Watt-Hours Explained
Manufacturers quote mAh, Ah and Wh more or less at random. One formula converts them all, and it is worth two minutes of your life.
The short answer
Watt-hours (Wh) = amp-hours (Ah) x volts (V). A 20,000mAh power bank is 20Ah at a typical 3.7V cell voltage, so 20 x 3.7 = about 74Wh. That is why a power bank and a portable power station are not comparable on the mAh number: a 288Wh power station holds roughly four times the energy of the biggest power bank you can buy.
Why three different units exist at all
Amp-hours measure charge, not energy. They only tell you something useful if you also know the voltage. Phone accessory makers quote mAh because the numbers look big — 20,000 reads better than 74. Power station makers quote watt-hours because it is the only unit that lets you compare across different battery voltages, and because it is the unit that maps directly onto appliance ratings.
Watts are already volts x amps, so watt-hours are volts x amp-hours: energy, not charge. It is the same unit your utility bills you in, just a thousand times smaller (1kWh = 1,000Wh).
The conversions, in both directions
| Quoted as | Conversion | Result |
|---|---|---|
| 10,000mAh power bank | 10Ah x 3.7V | 37Wh |
| 20,000mAh power bank | 20Ah x 3.7V | 74Wh |
| 27,000mAh power bank | 27Ah x 3.7V | About 100Wh — the airline limit |
| 100Ah 12V leisure battery | 100Ah x 12V | 1,200Wh |
| 100Ah 12V, usable at 50% (lead-acid) | 1,200Wh x 0.5 | 600Wh usable |
| 100Ah 12V LiFePO4, usable at 90% | 1,200Wh x 0.9 | 1,080Wh usable |
Those last two rows are the reason a 100Ah lithium battery is not simply a more expensive 100Ah lead-acid battery. Lead-acid is damaged by deep discharge, so convention is to use only half of it. LiFePO4 will give you most of its capacity without harm. Same nameplate, nearly double the usable energy. LiFePO4 vs NMC covers the chemistry side.
Turning watt-hours into runtime
Runtime in hours is roughly capacity in watt-hours divided by the device's draw in watts — then reduced, because converting the battery's DC to household AC costs energy.
For anything with a compressor, that arithmetic changes again, because the device does not run continuously. A fridge rated 45W running at a 40% duty cycle averages 18W, not 45W. Getting duty cycle wrong in either direction is the second most common sizing error after ignoring surge — what size do I need works it through properly.
Where the number on the box comes from
A manufacturer's stated capacity is the nominal cell capacity, measured under favorable conditions. You will not get all of it out of the AC outlets, because the inverter takes its cut and because the battery management system keeps a reserve. Independent testers have consistently found delivered AC energy below nameplate: OutdoorGearLab, which does buy and instrument its units, reported that none of the products it tested delivered usable AC output matching claimed capacity. We have not measured this ourselves, and we are citing their finding rather than presenting it as ours — but it is the right expectation to carry into a purchase.
The one place the number is a legal limit
Watt-hours are also how aviation regulators classify batteries. The thresholds are 100Wh and 160Wh, and they apply to the battery's rating regardless of physical size. That is the entire reason a 74Wh power bank flies and a 288Wh power station does not. Can you bring a power station on a plane sets out the rules with the TSA and FAA citations.
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