Ah to Wh converter

This is the conversion that makes batteries comparable. Amp-hours describe charge and say nothing about voltage, so a 100 Ah 12 V battery and a 100 Ah 24 V battery have identical headline figures and hold 1,200 Wh and 2,400 Wh respectively. Any comparison that stops at amp-hours is comparing the wrong thing, and it is the reason specifications are increasingly quoted in Wh.

For sizing, watt-hours is also the unit that matches your loads. Appliances are rated in watts, so a 60 W load for 5 hours is 300 Wh, and the arithmetic is direct. Doing the same in amp-hours means converting every load through the system voltage first, which is more steps and more places to go wrong.

The usable figure applies a depth of discharge, defaulting to 80% for lithium. It is worth stating what that does not include: inverter efficiency (another 10–15% for AC loads), temperature derating (a lead-acid battery at 0 °C delivers perhaps 80% of its rated capacity), and ageing (most chemistries are specified to 80% capacity at end of life). Stacking those, a nominal 1,200 Wh bank might deliver 700 Wh of AC energy on a cold day in its fifth year — which is the number to size against if the system has to work.