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Lighting and electronics

Watt-hours (Wh)

Also called Wh, watt-hour, mAh vs Wh, battery capacity

What are watt-hours on a power bank?

Watt-hours measure how much energy a battery stores: voltage multiplied by capacity in amp-hours. Unlike mAh, which is quoted at the cell's own voltage, watt-hours let you compare any two batteries fairly, and they are the unit airlines use to set carry-on limits.

Watt-hours (Wh) at a glance

Formula
Wh = mAh x volts / 1,000
10,000 mAh bank
About 36 to 37 Wh
Carry-on limit
100 Wh without approval

How do you convert mAh to watt-hours?

Multiply the mAh figure by the battery's voltage and divide by 1,000. Power banks use lithium-ion cells at a nominal 3.6 to 3.7 volts, so a 10,000 mAh bank stores about 36 to 37 watt-hours, and a 20,000 mAh bank about twice that.

Hold that up against your phone. Most phone batteries store around 13 to 19 watt-hours, which is why a 10,000 mAh bank comes out to roughly one and a half to two charges once conversion losses are counted.

Why does a power bank deliver less than its rating?

The mAh on the box is measured at the cell's voltage, but USB charges at 5 volts or more, so the bank has to step the voltage up, and that conversion loses energy as heat. Expect roughly 60 to 70 percent of the rated capacity to reach your phone.

Some makers publish both numbers. Nitecore rates its Carbon Battery 6K at 6,000 mAh at the cell but 3,700 mAh at its 5 volt output, which is the more useful figure.

Why do airlines care about watt-hours?

Because energy, not mAh, is what makes a battery a fire risk. The FAA allows spare lithium-ion batteries and power banks up to 100 watt-hours in carry-on bags without approval, between 100 and 160 watt-hours with airline approval and at most two per person, and never in checked bags.

What this means when you're choosing

  • A watt-hour figure printed on the case, which airlines check
  • A rated capacity at 5 volts, which is what your phone actually receives
  • Watt-hours per ounce when comparing two banks for a pack

The common mistake

Comparing batteries by mAh across different voltages

A 3,000 mAh pack of 12 volt cells stores far more energy than a 3,000 mAh phone battery. mAh only compares batteries at the same voltage; watt-hours compare anything.

Common questions

How many watt-hours is a 10,000 mAh power bank?

About 36 to 37 watt-hours, since the lithium-ion cells inside run at a nominal 3.6 to 3.7 volts. Check the case: most banks print the exact figure.

Can I fly with a 20,000 mAh power bank?

Yes, in your carry-on. A 20,000 mAh bank stores about 72 to 74 watt-hours, under the FAA's 100 watt-hour line, so it needs no airline approval. It can never go in checked baggage.

Where this is defined

Where this comes up

Guides on Tom's Hiking where watt-hours (wh) changes which product is right for you.

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