How to Convert Watt-Hours to Milliamp-Hours

Units such as watt-hours (Wh) and milliampere-hours (mAh) are often used to describe battery capacity. You may want to convert Wh to mAh, and we’ll go over how to do that below.

Although it is impossible to measure the internal characteristics of a battery because it is a closed system, we can use the observable parameters at the battery’s terminals to estimate internal measurements, such as the stored charge and total energy capacity.

The watt-hour capacity of a battery, expressed as Wh or W·h, is a measure of the amount of energy being supplied to the load. It is a measure of the total electrical power the battery can supply for an hour or the total duration for which it can supply a specific load.

The milliampere-hour capacity of a battery describes the duration for which a battery can supply one milliampere of current and the maximum amount of current it can supply for one hour.

For example, a 10 mAh of capacity means that the battery can supply a load of 10 milliamperes for one hour before losing all its charge, or it can indicate that the battery can supply a 1 milliampere current to a load for 10 hours. The electric charge for smaller batteries is generally measured in milliamp-hours, abbreviated mAh or mA·h.

To convert watt hours to amp hours, simply divide watt hours by volts:

Amp hours (Ah) = Watt hours (Wh) ÷ Volts (V)

Example

Let's say you have a 2000Wh LiFePO4 battery. You know it's a 12 volt battery, and you want to calculate its amp hours.

To do so, you'd divide watt hours by volts:

2000Wh ÷ 12V = 166.67Ah

In this example, your battery has a 166.67Ah capacity.

Note: Most people use a battery's nominal voltage when calculating amp hours or watt hours, but it's worth reminding you that a battery's actual voltage is slightly different and varies based on its remaining capacity. For instance, a 12V LiFePO4 battery actually has a voltage of 13.6V when fully charged.