kWh, an acronym for kilowatt-hours, is a standard unit of measurement used to determine how long a battery will power an electronic device before it needs to be recharged. To quickly find out if one lead acid battery will last longer than another, compare the kilowatt-hour ratings. The one with the higher value will provide power for a longer time. Most lead acid batteries specifications do not list kilowatt-hour capacity. Instead they list ampere-hour capacity. The kilowatt-hour rating of the lead acid battery can be calculated from the ampere-hour rating and the voltage rating of the battery.
Find out the ampere-hour rating of your battery. Obtain the manufacture's data sheet for the battery. It should list the number of ampere-hours that at which the battery is rated. Remember though that this ampere-hour rating is for a given set of conditions that the manufacture specifies. So examine the battery's data sheet for the ampere-hour ratings that relate directly to your battery's operating environment, such as temperature.
Find out the voltage rating of your lead acid battery. Look on the manufacturer's data sheet or on the battery. There may be different voltage ratings for different operating temperatures. Use the voltage rating that corresponds to the temperature of the environment that your battery is most often in.
Calculate the watt-hours. The number of watt-hours is equal to the number of ampere-hours multiplied by the voltage rating of the battery. If a lead acid battery has a 7 ampere-hour rating and a voltage rating of 12 V, the battery's watt-hour rating would be 84 watt-hours, since 7 times 12 is 84.
Calculate the kWh. 1 kilowatt-hour equals 1000 watts. To convert watt-hours to kilowatt-hours, divide watt-hours by 1000. For the result obtained in Step 3, 84 watt-hours equals 0.084 kilowatt-hours, since 84 divided by 1000 equals 0.084.
The ampere-hours rating of a battery depends on how frequently it is used, how long the battery is used each time, the actual temperature the battery is operated at and how old the battery is. Generally, the battery capacity, that is the ampere-hour rating, will be lower if you use the battery continuously for a long period of time, the device you power requires a large amount of current and the battery operates at lower temperatures. Once you know the kWh rating, you can roughly determine how long a lead acid battery will last as long as you know how much power the devices attached to it require.