How to Calculate Battery Watt-Hours

••• battery image by Aditia Patria Warman from

If you're a recreational vehicle owner, knowing how long your battery will provide power is important. And that's because your battery will powers all your electrical appliances and electronic devices.

So, to prevent a miniature blackout when you least expect it, learn to calculate how long your battery will last before it needs to be recharged. To do that, total the watt-hours that all your appliances and devices use. Once you know you that number and your battery's watt-hour rating, you will have the information you need to prevent surprise power outages.

A watt is defined as one joule per second and is the standard unit of energy in physics. Consequently, multiplying power by a unit of time restores units of energy, and in modern societies, either the watt-hour (W-h) or the kilowatt-hour (kW-hr) is the most often used. You may at times need to convert at times between maH and wH (milliamp-hours and W-h), with an ampere being the standard unit if electrical current.

    Determine and list the watt rating of each of the device that your battery powers. Watt ratings of appliance and electronic device are often listed on the product. This rating can be used as the watt rating for the device. But if not given, typical watt ratings for specific electronic devices can be found on the Department of Energy's website.

    However, for a precise measurement a watt meter may need to be used. This is because the actual power your device consumes will not be the same as the specification listed on the device, and it will not be same as what the Department of Energy provides. Those ratings are only rough estimates.

    Estimate the time each of your devices is on over a given time period. The actual energy your device consumes is a function of the amount of time it is on and the number of watts is uses. For each device, estimate how long the device is on, on average, over a given time period; for example, each day. For example, your PC may, on average, be on for three hours per day.

    Calculate the watt-hours for each electronic device. For each device, multiply the device watt rating by the average number of hours the devices is on for the set time period. For example, if your PC was rated at 20 watts and was on three hours per day, it would use 60 watt-hours of energy per day.

    • Calculating kilowatt-hours is as easy as calculating watt-hours. To get from kW-hr to W-hr, multiply by 1,000; to go from W-hr to kW-hr, by far the more common unit, divide by 1,000.

    Add up the watt-hours for each of the devices. The total number of watt-hours that your battery will need to supply over the set time period is equal to the sum of the watt-hours that each of your devices uses. For example, if your PC required 5 watt-hours per day and your fan required 5 watt-hours per day, you would need 10 watt-hours of energy per day,

    Further, if your electronic devices required 10 watt-hours per day and all of these devices needed to be powered for 10 days, the total watt-hours your battery must supply over the 10-day period would be 100 watt-hours--that is, 10 multiplied by 10. This means that you would need a battery that was rated at 100 watt-hours to ensure that you had power over the 10-day period.

    Things You'll Need

    • Watt specfications for appliances and electronic devices
    • Watt-hour rating of batteries
    • Internet connection
    • Pencil
    • Paper


    • There are a number of watt-hour calculators on the web that make it easy to determine total battery watt-hours needed. They use drop-down menus to select the type of appliance, and then they let you specify the average number of hours you use the device per day. They then calculate the watt-hours for each device and then sum them.

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