Before you buy an inverter or a battery, it helps to know how long a battery will really run your devices. A 100Ah battery sounds large, but at a 1000W load it can be empty in well under an hour. This calculator gives you a realistic estimate.
Enter your battery voltage and capacity, choose the battery type, add your load in watts and press the button. You will see the runtime, the usable energy and the current the battery must deliver.
Battery runtime calculator
How the battery runtime calculator works
A battery stores energy in watt-hours. We find it by multiplying the voltage by the capacity in amp-hours, so a 12V 100Ah battery holds about 1,200 Wh. You cannot use all of it. Lead-acid and AGM batteries last much longer if you only use about half, while lithium LiFePO4 batteries can safely give around 90 percent.
The calculator multiplies the usable energy by the inverter efficiency, because an inverter wastes some power as heat. Most good inverters are around 85 to 92 percent efficient. It then divides by your load in watts to get the hours. Runtime equals usable watt-hours times efficiency, divided by load watts.
Ways to make your battery last longer
Lower the load
Switch off devices you are not using. LED lights and laptops use far less than heaters, kettles and hair dryers. The load is the biggest factor in runtime.
Choose the right battery type
Lithium batteries give more usable energy per amp-hour and handle heavy loads better. They cost more at first, but for regular use they often work out cheaper over time.
Use a higher voltage for big loads
A 24V or 48V battery bank draws fewer amps for the same power. That means thinner cables, less heat and less waste. Our cable and fuse calculator shows the current for each voltage.
Frequently asked questions
How long will a 100Ah 12V battery run a 500W load?
A lead-acid or AGM battery will run a 500W load for roughly 1 hour, because you should only use about half of its 1,200 Wh. A lithium LiFePO4 battery of the same size gives around 1.8 hours.
Why does my real runtime come out shorter?
Heavy loads, cold weather, an old battery and high current draw all reduce capacity. Treat the result as a best-case figure and keep a safety margin.
Can I add batteries to run longer?
Yes. Batteries of the same voltage, type and age can be connected in parallel to add capacity. Enter the number of batteries in the calculator.
What depth of discharge should I use?
About 50 percent for lead-acid and AGM, and up to 90 percent for LiFePO4. Using more than that shortens the life of the battery.
Please note: Runtime is an estimate. Battery capacity changes with temperature, age and discharge rate.