Formula
Usable energy is nameplate capacity times the depth of discharge:
hours = capacity kWh × depth / load kW
The small-battery runtime page uses milliamp-hours and a cell voltage. This page starts from kilowatt-hours.
Assumptions
- The load stays at the watts you enter.
- The depth of discharge is the share of nameplate capacity you are willing to use.
- Inverter loss is not applied again. Lower the depth if you want to reserve it.
Worked example
A 10 kWh battery used to 80% depth, carrying 500 watts, lasts 16 h.
Input definitions
- Depth of discharge
- The percent of nameplate capacity that may be used. 80% leaves 20% untouched.
- Load
- The steady watts the battery must supply.
Limitations
This is a planning estimate. The rate, sun hours, and losses are numbers you type. It is not a utility bill, a solar design, or a room-by-room load calculation.
Questions
Why not enter amp-hours?
A bank of 12-volt batteries counted from amp-hours is the backup battery page. This page is for a kilowatt-hour pack.
Does this include solar charging during the outage?
No. The hours assume the pack is not being refilled.