The formula
Efficiency is 1.0 for DC loads. For anything running through an inverter, use around 0.85–0.9, because the inverter turns some of the energy into heat.
Worked example
A 100 Ah 12 V AGM battery runs a 60 W DC fridge. Using only 50% of the battery: 100 × 12 × 0.5 ÷ 60 = 10 hours of compressor run time. Because a fridge compressor cycles on and off, the battery will usually last much longer than 10 hours in real use. Use the fridge's average daily consumption in the power consumption calculator for a better daily estimate.
Depth of discharge by battery type
| Battery type | Typical usable depth of discharge |
|---|---|
| Flooded lead-acid leisure battery | 50% |
| AGM / gel | 50% (up to 60–80% occasionally) |
| LiFePO4 (lithium iron phosphate) | 80–100% (90% is a good planning figure) |
Why real run times are often shorter
- High currents reduce lead-acid capacity. A 100 Ah rating is at the 20-hour rate (5 A). At 25 A you may only get 70–80 Ah. See Peukert's law.
- Cold weather reduces available capacity, and most lithium batteries can't be charged below 0 °C.
- Old batteries lose capacity with age and cycles.
Frequently asked questions
How long will a 100Ah battery run a 100W load?
At 12 V with 50% usable (lead-acid), about 6 hours. With a LiFePO4 battery using 90%, about 11.5 hours on DC, or roughly 10 hours through an inverter.
How long will a 12V battery run a 1000W inverter?
A 1,000 W load draws about 95 A from a 12 V battery once inverter losses are included. A 100 Ah LiFePO4 would last under an hour; a 200 Ah bank about 1.7–1.9 hours.
Should I use the battery's Ah or Wh rating?
Either works. If you know Wh, enter it as Ah × V, or use the Ah to Wh converter first.