Battery Runtime Calculator
Enter the size of your battery and the load you want to run. The calculator estimates how many hours the battery will last after depth of discharge and inverter losses.
How to use this Battery Runtime Calculator
- Enter the capacity of the battery in amp-hours and its voltage.
- Enter the load you want to run in watts.
- Enter the depth of discharge you are willing to use.
- Enter the inverter efficiency, or 100 if the load runs on DC directly.
- Press Calculate to see the runtime.
How to calculate battery runtime
Runtime is the usable energy in the battery divided by the load.
- Battery energy (Wh) = amp-hours x volts
- Usable energy = energy x depth of discharge x inverter efficiency
- Runtime (hours) = usable energy / load watts
- Battery current = load / efficiency / volts
Worked example
A 12 V, 100 Ah battery holds 1,200 Wh. If you plan to use 80% of it and the inverter is 90% efficient, the usable energy is 864 Wh. A 300 W load then runs for 864 / 300 = 2.88 hours, which is 2 hours and 53 minutes. The battery supplies about 27.8 A at that load.
If you double the load to 600 W, the runtime halves to 1.44 hours. Two batteries wired in parallel double the amp-hours, and the runtime doubles too. Batteries often give less than their rating at high current, in the cold or when they are old, so leave extra room. Always check that the wires, fuses and the inverter can carry the current that the battery supplies. To plan for a longer outage, work out the total watt-hours of the loads you want to keep on, add a margin of 20 percent or more for ageing and cold weather, and compare it with the usable energy shown here. If the gap is large, you can add batteries in parallel, cut the load, or add a charging source such as solar panels or a generator.
Assumptions & limits
- The calculation uses a simple energy model. It ignores the Peukert effect, temperature and ageing, which shorten runtime for many batteries.
- Depth of discharge and inverter efficiency are inputs from your data sheets.
- The load is a constant figure in watts. Real loads change.
- It does not include the energy used by the inverter itself when idle.
- Check wire and fuse ratings for the current shown. Check the National Electrical Code and your local rules, and use a licensed electrician for real installations.
Frequently asked questions
How do I calculate battery runtime?
Multiply amp-hours by volts to get watt-hours, apply depth of discharge and efficiency, then divide by the load in watts.
What is depth of discharge?
The share of the battery capacity you actually use.
Why is my battery shorter than the estimate?
Cold, age, high current and extra losses all reduce real capacity.
Does wiring batteries in parallel increase runtime?
Yes. Parallel batteries add amp-hours at the same voltage.
Do I need to include the inverter?
Yes, if the load runs on AC. Inverters waste some energy.
How do I convert watt-hours to amp-hours?
Divide watt-hours by the battery voltage.
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Last updated: November 2026