Free tool
How long will the battery last?
Enter the battery and what you want to run. You get the usable energy, the
runtime, and the current the battery has to deliver.
of runtime at this load
The calculation. Usable energy is the nameplate capacity times
the depth of discharge. Runtime is that usable energy times the inverter efficiency, divided by
the load. With Ah, the nameplate energy is Ah times the battery voltage, divided by a thousand.
Why the real number is lower. Cold weather, an ageing battery,
the standby draw of the inverter and short bursts from motors all take a bite. Treat the result
as the best case and plan with roughly ten to twenty percent less.
Some loads are not what the label says. A fridge or a well pump draws
its rated power only part of the time, but starts at several times that. Sizing a battery for a
whole house needs a real load list, not one number.