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On my van, 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. The 100W load at 12V draws 100/12 ≈ 8.33A. So:
100Wh / 12V = 8.33A (current draw)
1 watt-hour is 1 watt of power expended for 1 hour.
Since the battery is 100Ah and we’re drawing about 8.33A, I can run it for 12 hours (100Ah / 8.33A ≈ 12h).
If I had a device that consumed 200W, I could run it for 6 hours.
This assumes the battery is at 100% and we can use 100% of its capacity, which is almost the case with a LiFePO4 battery, but not with regular lead-acid batteries.