How long will it last tonight?

The question everyone asks, with the arithmetic that answers it.

Step 1: Convert to watt-hours. Amp-hours × voltage. This is why comparing amp-hours across voltages is meaningless — always convert to watt-hours to compare fairly.

Step 2: Adjust for what's usable. LiFePO₄ gives you nearly all of it — call it 90–100%. Lead-acid gives about 50% before you damage it.

Step 3: Total your load in watts. Everything running simultaneously.

Step 4: Divide. Usable watt-hours ÷ watts = hours.

Worked example: 2,160Wh usable, running a 50W fridge (duty-cycled to ~25W average), 10W of lights, and a 60W laptop = 95W. 2,160 ÷ 95 ≈ 22 hours.

The adjustments that make it real: AC loads through an inverter cost about 10–15% more than the appliance's rating. Inverter idle draw counts. Cold weather reduces capacity. Batteries lose capacity over their life. Duty cycles matter.