Key takeaways
- Usable energy = Ah × V × (DoD ÷ 100); apply inverter efficiency before dividing by load.
- Runtime (h) = usable Wh × efficiency ÷ load W — so doubling the load roughly halves the hours.
- Use ~50% DoD for lead-acid, 80–100% for LiFePO₄.
- A 200 Ah / 12 V battery at 50% DoD and 90% eff runs a 300 W load about 3.6 h (3 h 36 min).
How to calculate battery backup runtime
Runtime is just stored energy divided by how fast you spend it. Start with the battery's total energy — amp-hours times voltage — then keep only the share you can safely use (depth of discharge) and subtract inverter losses. Divide the energy that actually reaches your AC load by the load's wattage and you have the hours it will run.
Two factors quietly cut runtime: depth of discharge and inverter efficiency. A lead-acid battery should only give up about half its rated capacity, and the inverter loses another 10% turning DC into AC — so a 200 Ah battery delivers far less than its nameplate suggests.
Worked example: 200 Ah, 12 V, 50% DoD, 90% eff, 300 W
Battery energy = 200 × 12 = 2,400 Wh. Usable at 50% DoD = 2,400 × 0.5 = 1,200 Wh. After a 90% inverter = 1,200 × 0.9 = 1,080 Wh delivered. Divide by a 300 W load: 1,080 ÷ 300 = 3.6 hours, or 3 h 36 min. Switch to LiFePO₄ at 80% DoD and the same battery runs roughly 5.8 hours.
Runtime by load (200 Ah / 12 V, 50% DoD, 90% eff = 1,080 Wh usable)
| Load | Runtime (hours) | Runtime (h:min) |
|---|---|---|
| 100 W | 10.8 h | 10 h 48 min |
| 300 W | 3.6 h | 3 h 36 min |
| 600 W | 1.8 h | 1 h 48 min |
| 1000 W | 1.1 h | 1 h 5 min |
Start with an honest load, then size the bank
Runtime is only as good as your load figure — total your appliances first, then check the bank. Size the battery itself with the battery bank sizing calculator, and build an accurate watt figure with the off-grid load calculator. Choosing lithium or a deeper-cycle chemistry extends runtime without buying a physically larger battery.
Frequently asked questions
How long will my battery last?
Runtime (h) = Ah × V × (DoD ÷ 100) × (eff ÷ 100) ÷ load W. A 200 Ah 12 V battery at 50% DoD, 90% eff runs a 300 W load about 3.6 h (3 h 36 min).
How do I calculate backup runtime?
Find usable energy (Ah × V × DoD), multiply by inverter efficiency, then divide by load watts. The decimal part × 60 gives minutes.
Why does depth of discharge matter?
You can only safely use part of the rated capacity — ~50% for lead-acid, 80–100% for LiFePO₄. Lower usable DoD means less runtime for the same Ah.
What is inverter efficiency loss?
Inverters lose 8–15% converting DC to AC. A 90% inverter delivers only 90% of stored energy, so apply efficiency before dividing by the load.
Lead-acid vs lithium runtime?
For the same Ah rating, LiFePO₄ lasts longer — it allows 80–100% DoD versus ~50% for lead-acid, roughly 1.6–2× the usable runtime.
Will a higher load drain it faster?
Yes — runtime is inversely proportional to load, so doubling watts halves the hours. Lead-acid drains faster still at high loads due to the Peukert effect.
Usable-capacity and inverter-efficiency guidance follows lithium and lead-acid manufacturer data — see Battle Born Batteries. The Wh, DoD, and runtime relationships here are exact arithmetic.
Last reviewed June 2026