# Calculating Ah from Watts

> Learn how to calculate how long a 12V battery can power a device, converting watts to amp-hours with the Wh / V formula using a real van life example.

Author: [Flavio Copes](https://flaviocopes.com/about/) | Published: 2021-08-03 | Topics: [Arduino](https://flaviocopes.com/tags/electronics/) | Canonical: https://flaviocopes.com/calculating-ah-from-watts/

On my [van](https://flaviocopes.com/van-life/), I have a 12V 100Ah battery.

Let's say I have a 100W hair dryer that I run through a power inverter. For how many hours can I run it, before the battery capacity ends?

The formula to use is `Wh / V`, which in this case means

100Wh/12V = 8,333Ah

> 1 watt-hour is 1 watt of power expended for 1 hour

Since the battery is 100Ah, I can run it for 12 hours (100Ah / 8,333Ah).

If I had a device that consumed 200W, I could run it for 6 hours.

This assuming the battery is 100% and I can consume 100% of its power, what almost happens with a LiFePo4 battery, but not for regular lead-acid batteries.