How to Calculate Battery for Lights
- 1). Look at the electrical rating on each of the lamps. Write down the wattage rating of each lamp.
- 2). Add the two wattage ratings together, and divide by the voltage rating. As an example, for two 12-volt lamps each rated at 24 watts, divide 48 watts divided by 12 volts (which is 4 amps).
- 3). Multiply how many amps will be needed for a specified period of time. For example, if the electrical circuit in the example above must be on for one hour, multiply 1 hour by 4 amps to get 4 amp-hours.