How to calculate battery runtime
Convert the battery's amp-hours to watt-hours, keep only the portion you can safely use (depth of discharge), subtract inverter losses if you power AC devices, and divide by the load in watts.
Example: 100 Ah 12 V LiFePO4 running a 100 W load
100 × 12 = 1,200 Wh. × 90% usable × 90% inverter efficiency = 972 Wh. 972 ÷ 100 W = 9.7 hours. The same size lead-acid battery gives about 5.4 hours because only 50% is usable.
Usable capacity by battery type
| Type | Recommended depth of discharge | Typical cycle life |
|---|---|---|
| LiFePO4 (lithium iron phosphate) | 80–100% | 3,000–6,000 |
| Lithium-ion NMC | 80% | 1,000–2,000 |
| AGM / gel | 50% | 400–1,000 |
| Flooded lead-acid | 50% | 300–1,000 |
Typical loads
LED light 5–10 W · Laptop 50–90 W · 12 V fridge 40–60 W average · CPAP 30–60 W · TV 60–150 W · Microwave 1,000–1,500 W.
Frequently asked questions
How long will a 100Ah battery last?
A 12 V 100 Ah LiFePO4 battery runs a 100 W load for about 9–10 hours through an inverter, or about 5 hours if it is lead-acid.
Why can't I use the full capacity of a lead-acid battery?
Discharging lead-acid below 50% sharply shortens its life. Lithium batteries tolerate deep discharge much better.
Does a DC load need the inverter setting?
No. Uncheck the inverter option for 12 V devices powered directly from the battery.