Ampere-hours, or amp-hours, are normally used to describe the energy capacity of batteries, although they can be used to calculate the required load for any electrical scenario. To calculate amp-hours from watts, you need to use equivalent terms: either the number of watt-hours or how long something operates at a certain number of watts. Couple this with voltage, and you can easy calculate amp-hours.
Multiply the number of watts by the number of hours to calculate watt-hours. As an example, if you know your laptop computer uses 30 watts of power and you want a battery to power it for four hours, you need 120 watt-hours of stored energy.
Divide this figure by the energy efficiency of the battery to compensate for the lost energy. The battery's efficiency is listed in its specifications. In the example, if the battery has an 85 percent efficiency, you divide 120 by 0.85 to derive a required 141 watt-hours. If you wish to calculate only theoretical values, you can skip this step.
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Divide this figure by the voltage to calculate amp-hours. Voltage is also listed in the battery's specifications. In the example, if the battery is rated at 19 volts, divide 141 watt-hours by 19 volts. This tells you your new battery needs at least 7.42 amp-hours to run your laptop for four hours.