mAh and Wh: how to work out battery capacity properly

mAh (milliamp-hour) and Wh (watt-hour) are units that measure how much energy a battery holds. mAh shows the charge at a particular voltage, while Wh is the energy itself, which makes it better for comparing different devices.

mAh and Wh: volume plus voltage "height" together make energy

In plain words

Imagine two buckets of water. One holds 10 litres, the other 8, but the second sits on the first floor, so its water comes out with more pressure. Which is "stronger"? Litres are mAh, the height is voltage, and the real "energy of the water" is Wh. Comparing batteries only by mAh is like comparing buckets without looking at how high they are.

How it works

Capacity in mAh shows what current a battery can deliver for an hour.
To get energy, multiply mAh by voltage and divide by 1000: Wh = mAh × V / 1000.
A phone battery is about 3.85 V: 5000 mAh × 3.85 / 1000 ≈ 19 Wh.
A 20,000 mAh power bank is usually rated at its cells' 3.7 V - about 74 Wh.
Some energy is lost to voltage conversion and heat, so noticeably less reaches the phone than the power bank's label suggests.

Why a power bank gives a phone less energy than its label says

Where you'll see it

You see mAh in the specs of phones, tablets, watches and power banks - in our tables it's the "Battery" row. Wh appears on laptops, power stations and in small print on power bank casings. Wh also matters at the airport: power banks up to 100 Wh are usually allowed in carry-on luggage.

Does it matter when buying

Yes, especially when choosing a power bank or power station for power cuts. I suggest this rule of thumb: multiply the power bank's Wh by about two-thirds (conversion losses) and divide by your phone's Wh. That's the real number of charges. And remember: the same mAh doesn't mean the same battery life - a lot depends on the screen and chip.

Myths

Myth: A 20,000 mAh power bank will charge a 5000 mAh phone four times. Reality: Usually only two or three times: the voltages differ and some energy turns into heat.
Myth: More mAh always means longer battery life. Reality: Not always. An efficient chip and screen with a smaller battery can last longer.

FAQ

How do I convert mAh to Wh?

Multiply mAh by the battery voltage and divide by 1000. For phones, use about 3.85 V.

Why do laptops list Wh?

Because their batteries use a different voltage, and Wh gives a fair energy comparison.

How many Wh can I take on a plane?

Usually up to 100 Wh without the airline's approval. Check your carrier's exact rules.