# How to calculate watt hours gained charging battery?

I need an estimate of how many watt hours a battery has gained after charging to a certain number of volts.

With a 12 V, 50 Ah lead-acid car battery, charged from 12.5 V to 13.0 V, is it as simple as this?:

(13.0 V - 12.5 V) * 50 Ah = 25 Wh

This only needs to be an estimate. The battery powers a load with a constant draw that turns on and off 5-20 times per day, so I know how long it can run on a given number of watt hours, and just need an idea of:

'If I charged this many volts, I can expect this many extra watt hours available for spending'.

• Not like that because that equation implies that the battery is dead when it reaches 0V but the batter is dead long, long before that. If you're only going off voltage, you'll just need to find and use charts for this. SOmething like this: powertechsystems.eu/home/tech-corner/… Mar 16, 2022 at 21:22
• If you can manage to add a "smart shunt' to your system you will get much better results. Otherwise using a chart like DKNguyen said will give you a very rough estimate. Some things to expect if you do this are that the estimate will sometimes be off. Also, the battery voltage is going to go up and down with the load. Higher when load is off, and lower when load is on. This can give a false reading. Mar 17, 2022 at 4:02