Battery Runtime Calculator
Works out how long a battery will actually run a load. The nameplate figure is not the answer — depth of discharge and inverter efficiency between them often halve it.
Share sends a link that reopens these numbers. Copy details pastes the full breakdown as a list.
The formula
hours = (Ah × V × depth of discharge × efficiency) ÷ watts
Nameplate capacity is not usable capacity. Lead-acid gives up half of it if you want the battery to last; an inverter takes another tenth converting DC to AC.
What to watch out for
Three reasons the simple division is wrong:
- Depth of discharge. Taking a lead-acid battery below 50% repeatedly destroys it within months. LiFePO₄ tolerates 90%. That single factor is nearly a 2× difference in real capacity for the same nameplate rating.
- Inverter losses. Converting 12 V DC to 230 V AC costs 8–12%. A DC load skips this entirely — set efficiency to 100%.
- Peukert's effect. Batteries deliver less total capacity at high discharge rates. A lead-acid pack drained in one hour may give only 60% of its 20-hour rating. This calculator does not model that, so treat fast-discharge answers as optimistic.
Frequently Asked Questions
How long will a 100 Ah battery run a 200 W load?
About 4.9 hours with LiFePO₄ at 90% depth of discharge through a 90% efficient inverter. With AGM at 50% it is about 2.7 hours — same battery size, half the runtime.
Why can I not use the full capacity?
Discharging deeply shortens battery life sharply, and lead-acid especially. Manufacturers rate cycle life against depth of discharge; 50% is the usual compromise for lead-acid, 80–90% for lithium.
Do I need to account for the inverter?
Yes for any mains-voltage load — it costs about 10%. For a DC load such as 12 V lighting or a DC fridge, set efficiency to 100%.