Inverter and battery runtime

How this runtime is built

A battery stores amp-hours. An inverter turns that into alternating current and wastes some of it as heat. Multiply amp-hours by voltage and you have watt-hours. Multiply again by the share of the battery you are willing to use, and by the efficiency, and you have watt-hours you can spend. Divide by the load and you have hours.

The headroom step is the one listings skip. A 65 watt laptop on paper is not a reason to buy a 65 watt inverter. This page looks for a common continuous size at or above 125 percent of the load. Surge watts, the number printed in the largest type on the box, are not the number that matters for an hour of work.

Nothing here measures your cables, the cold, or the fridge that is already running. A starting battery that is also expected to crank the truck in the morning should not be planned down to the inverter’s low-voltage cutoff. Read the sheet for the inverter you actually own, then change the efficiency field.

Related: inverter guide and 12V coffee.