The key is to use the watts you know to calculate the amps at the battery voltage . For example, say you want to run a 250 watt 110VAC light bulb from an inverter for 5 hours. Amp-hours (at 12 volts) = watt-hours / 12 volts = 1470 / 12 = 122.5 amp-hours.

How many hours is 1 amp hour?

An amp-hour is one amp for one hour, or 10 amps for 1/10 of an hour and so forth. It is amps X hours. If you have something that pulls 20 amps, and you use it for 20 minutes, then the amp-hours used would be 20 (amps) X . 333 (hours), or 6.67 AH.

How do you calculate amp hours per day?

Determine the Average Daily Load

You listen to either the radio or cassette player for 6 hours each day. The 12 volt system you have is rated at 0.2 amps at 12 volts. Multiply the amps by the hours and you get the result of 1.2 amp-hours per day.