To calculate a battery's amp hours, divide its watt hours by its voltage. Formula:battery amp hours = battery watt hours ÷ battery voltage Abbreviated:Ah = Wh ÷ V Calculator: Watt. To calculate a battery's watt hours, multiply its amp hours by its voltage. Formula:battery watt hours = battery amp hours × battery voltage Abbreviated formula:Wh = Ah × V Calculator: Amp Hours to Watt Hours Calculator If your battery's capacity is given. To get a very roughestimate of how many amp hours your battery needs to have, you need to know: 1. Device current draw in amps (A): How many amps does the device you're powering.