Battery Runtime Calculator: How Long Will It Last?
If you want the short answer, battery runtime is just usable watt-hours divided by load, but usable watt-hours are always lower than the headline number.
The catch is people often buy from the sticker and ignore conversion losses, battery age and the fact that devices do not always draw a clean fixed load.
Need the runtime fast?
Use the Battery Runtime Calculator and plug in the battery size and device watts.
Use the Battery Runtime Calculator and plug in the battery size and device watts.
Worked example
A 500Wh battery with about 85% usable energy gives you roughly 425Wh usable. Running a 60W device, that is about 7 hours on paper.
Bottom line
The maths is simple once you stop pretending the sticker number is the real number. Use the calculator to get a rough answer quickly.