How to Calculate Amp Hours by Watt Usage

Converting from amp-hours to watts or vice-versa isn't as simple a process as you might think, because they measure different things. Amp-hours measure an amount of current, while watt-hours are a measure of electrical power. However, all you need to do is convert watts to watt-hours, and then you can use a simple equation to convert watt-hours to amp-hours. Before completing the calculation, you need to understand what the terms mean and find out the voltage of the battery.

An amp-hour (Ah) tells you the amount of current a battery can provide over a certain amount of time, and a rating (usually in amp-hours or milliamp-hours) is used to give you an idea of how long a battery can provide a given amount of current. Amp-hours are calculated by multiplying the number of amps (A) a battery provides by the discharge time in hours (h). So, if a battery provides 10 amps of current for 10 hours, it is a 10 amps × 10 hours = 100 Ah battery.

A watt (W) is a measure of power, with 1 watt representing one joule (a unit of energy) per second. So, if a light bulb uses 60 watts, it requires 60 joules of energy per second. In a way similar to amp-hours, a watt-hour (Wh) is the amount of energy used when something operates at a given number of watts for a certain number of hours. This means 1 Wh is the amount of energy used by something operating at 1 W for an hour, and 60 Wh is the amount of energy something would use operating at 60 watts for an hour.

Before you can do the main conversion, you need to convert the number of watts your equipment operates at to an amount in Wh. This is easy: Multiply the amount of watts something operates at by the number of hours it operates for. For example, a 60-watt bulb running for three hours equals 60 W × 3 h = 180 Wh.

You can always convert back to W from Wh by dividing the amount in Wh by the number of hours the device was running.

There isn't an equation for amp-hours to watts directly, but with watt-hours, you can make use of the simple relation:

Amp-hours × Volts = Watt-hours

So, to get amp-hours from watt-hours, you use:

Watt-hours ÷ Volts = Amp-hours

This means that you need to find the voltage as listed on the battery to complete the calculation. The voltage is always listed on the battery or in its specifications somewhere, or possibly in the manual for the device if you have one.

Using the example of the 60-watt lightbulb working for three hours from before (180 Wh) and assuming the use of a 12-volt battery, the conversion can be completed.

Divide the amount of watt-hours by the number of volts to convert: 180 Wh ÷ 12 V = 15 Ah. This is the equivalent of 15 amps of current running for one hour, or – as in this case – 5 amps of current for three hours.

If you don't want to perform the calculation yourself, you can use an online amp-hours calculator to handle it for you. However, the calculation is straightforward, so you can easily do everything you need on a basic calculator on a cellphone or computer without the need for a specialized tool.