How to use this calculator
- Enter or choose battery capacity.
- Enter or choose capacity in.
- Enter or choose battery bank voltage.
- Enter or choose usable depth of discharge.
- Enter or choose inverter efficiency.
- The backup time updates as you type. Use Copy result or Share to keep it.
Method
Wh = Ah × V (or kWh × 1000); Runtime (h) = Wh × DoD × Inverter efficiency ÷ Load (W); Battery current = Load ÷ Efficiency ÷ V
- Ah
- rated amp-hour capacity
- V
- battery bank voltage
- DoD
- share of capacity you allow to be used
- Load
- power drawn by the connected devices
Worked example
- Rated energy = 200 Ah × 12 V = 2,400 Wh.
- Usable = 2,400 × 0.50 × 0.90 = 1,080 Wh.
- Runtime = 1,080 ÷ 300 W = 3.6 h = 3 h 36 min.
A 12 V 200 Ah battery used to 50% through a 90%-efficient inverter runs a 300 W load for about 3 hours 36 minutes.
What the result means
Treat the result as a best case. At high currents, in the cold, or with an older battery, real runtime is shorter, so allow a margin for anything critical.
Assumptions and limits
- The load is constant for the whole runtime.
- The battery delivers its rated capacity at this discharge rate.
- Ignores the Peukert effect (lead-acid capacity drops at high currents), temperature and battery age.
- Inverter standby consumption is not included.
Questions
Why not use 100% of the battery?
Deep discharges shorten battery life, especially for lead-acid. Most systems cut off before the battery is fully empty.
How do I find my load in watts?
Add up the rated watts of the devices on backup, or read the output power from the inverter display.