Wirebook › Glossary › Watt-hour (Wh)
What is a watt-hour (Wh)?
In one sentence: A measure of how much energy a battery holds; unlike mAh it means the same thing for every battery, so it's the fair way to compare a power bank with a phone.
A little more
A watt-hour is an amount of energy: one watt of power kept up for one hour. A phone battery holds roughly 10 to 20 Wh, a tablet about 30 Wh and a laptop 50 to 100 Wh. You get it from the battery label by multiplying milliamp-hours by the voltage and dividing by 1,000, so 10,000 mAh at 3.7 V is 37 Wh.
Because watt-hours already include the voltage, they compare fairly between any two batteries. That's why airlines set their power bank limits in Wh (100 Wh in hand luggage without asking), and why a power bank's real number of phone charges is worked out in Wh, after taking off the third or so that turns into heat on the way.
See it in action
- BonusWhy does a full battery die in the cold?Your tablet said 45% and switched off on a snowy hill, then came back indoors.7 min · interactive
- BonusWhy do phones and computers get hot?Heat up a pretend phone with games, charging, a duvet or a sunny car, tug a pretend blanket to feel how insulation traps heat, and watch it slow down to protect itself.9 min · interactive
Do it for real
Free grown-up tools and guides on Tooltern, our sister site. They work in your browser and nothing is uploaded.