Sizing a battery and knowing how long an existing battery will last are two different questions. Our battery size calculator answers the first — how big a bank you need for your usage. This tool answers the second, which is the one people ask in the moment: "I have this battery and this load — how many hours do I actually get?" It's the question that matters during a power cut, when planning an off-grid night, or when deciding whether your bank can carry a specific appliance.
The simple version, and why it's wrong
The naive calculation is just capacity divided by load: a 5 kWh battery running a 500 W (0.5 kW) load looks like it should last 10 hours. But that number is always too optimistic, for three reasons: you can't safely use 100% of a battery, the inverter wastes some energy converting DC to AC, and higher loads slightly reduce usable capacity. A realistic estimate has to account for the first two, which is what this calculator does.
The formula
First, find the usable energy — not the rated capacity, but what you can actually draw:
Usable energy (kWh) = Capacity × Depth of discharge × Inverter efficiency
Then divide by the load to get the runtime:
Runtime (hours) = Usable energy (kWh) ÷ Load (kW)
A worked example
Take a 5 kWh lithium battery running a 500 W load:
- Usable energy = 5 × 0.90 (DoD) × 0.95 (inverter) = 4.28 kWh
- Runtime = 4.28 ÷ 0.5 = about 8.5 hours
So the real answer is 8.5 hours, not the 10 the naive math suggested. If that same battery were lead-acid at 50% depth of discharge, the usable energy would drop to about 2.4 kWh and the runtime to under 5 hours — from the same rated capacity. That gap is why battery chemistry matters so much.
Why depth of discharge is the big lever
Depth of discharge (DoD) is how much of the battery you can safely use without shortening its life. Lead-acid should only go to about 50%, while lithium (LiFePO4) handles 80-90% comfortably. This single factor often makes the difference between a bank that carries you through the night and one that taps out early. When comparing batteries, always compare usable energy, not rated capacity.
Getting the numbers right
If your battery is rated in amp-hours (Ah) rather than kWh, convert it first: kWh = (Ah × system voltage) ÷ 1000. And if you're not sure of your total load, our daily load calculator helps you add it up appliance by appliance. Once you know how long your bank lasts, the battery size calculator helps if you decide you need more capacity, and the off-grid system wizard sizes a complete system.
Frequently asked questions
How do I calculate how long my battery will last?
Take your usable energy — capacity in kWh times depth of discharge — account for inverter losses, then divide by your load in kW. The result is runtime in hours. A 5 kWh lithium battery at 90% DoD with a 95% inverter delivers about 4.3 kWh, so a 0.5 kW load lasts roughly 8.5 hours. This calculator does it automatically.
What is depth of discharge and why does it matter?
Depth of discharge is how much of a battery's capacity you can safely use. Lithium allows about 80 to 90 percent, while lead-acid should only be discharged about 50 percent. A lead-acid battery of the same rated capacity therefore delivers far fewer usable hours than lithium.
Why does my battery last less than its rated capacity suggests?
Three things reduce real runtime: depth of discharge (you can't use 100%), inverter and wiring losses (5 to 10 percent), and higher discharge rates slightly reducing usable capacity. A realistic estimate always applies depth of discharge and an efficiency factor.