1. Multiply the number of watts by the number of hours to calculate watthours. As an example, if you know your laptop computer uses 30 watts of power and you want a battery to power
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There is one amp in twelve watts, when you have twelve volts of electricity. That equation is written as amps = watts/volts. This conversion is always figured at a fixed voltage.
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Divide the watts by the voltage of the circuit.
http://wiki.answers.com/Q/How_do_you_calculate_wat...

1 Familiarize yourself with the quantities involved. To understand the relationship between watts and amps, you will need to understand 3 different units: watts, amps, and volts.
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