About the Battery Runtime Calculator
Knowing how long a battery will last is a basic question for camping, boats, solar systems, alarms and gadgets. This calculator converts the capacity into watt-hours, keeps only the part you can safely use, allows for the losses in an inverter or circuit, and divides by the power of the load. The result is the runtime in hours and minutes. Type the capacity, the voltage, the load in watts, the usable share and the efficiency and the battery runtime appears at once, with a few related figures underneath, and you can copy the result with one tap. The sums run in your browser, so nothing you enter is uploaded.
Updated October 2026
How to use the Battery Runtime Calculator
- 1Enter the capacity, the voltage, the load in watts, the usable share and the efficiency.
- 2Pick the units or options from the menus if the battery runtime tool offers them.
- 3Read the battery runtime in the large result box.
- 4Look at the extra rows for related figures.
- 5Press Copy result to keep the answer.
When you need the battery runtime
Campers use it to size a battery for a fridge or lights, boat owners to plan for a night at anchor, and home owners to estimate how long a battery backup will keep a router and a few lights going in a power cut.
The usable share depends on the battery chemistry. Lead-acid batteries last longer if you draw them down no more than about half. Lithium iron phosphate batteries can use 80 to 90 percent. Efficiency losses are small for DC loads and larger when an inverter turns the power into mains.
How the battery runtime is worked out
Runtime in hours = capacity in Ah × voltage × usable share × efficiency ÷ load in watts. A 100 Ah, 12 V battery holds 1,200 Wh. Using 80 percent of it with a 90 percent efficient inverter leaves 864 Wh, which runs a 100 W load for 8.64 hours, or 8 h 38 min. The battery supplies about 8.3 A to the inverter.
Every answer on this page comes from that one rule, so you can check any result by hand with a calculator. The figures shown underneath, such as other units, are conversions of the same answer.
- Ah or mAh
- Usable share and efficiency
- Hours and minutes shown
Mistakes to avoid with battery runtime
Real runtime is shorter in the cold, for old batteries and at high discharge rates, because the usable capacity drops. Treat the result as a best case, and keep a margin. The tool also assumes a steady load; a load that varies needs the average power.
On phone and computer
On a phone, type into the boxes one after another and read the battery runtime underneath; Copy result puts it on the clipboard for a message or a note. On a computer the inputs sit on the left and the answer on the right, so you can change one figure and watch the battery runtime move. The battery runtime calculator works the same on both. Work out the capacity with mAh to Wh Calculator and Wh to mAh Calculator, see how long a charge takes with Battery Charge Time Calculator, and find the running cost with Electricity Cost Calculator.
Features
- Lead-acid: about 50%
- Cold shortens runtime
- Use the average load
- Clear messages when the battery runtime inputs make no sense
- Copy the result with one tap
- Private: nothing is uploaded
Frequently asked questions
How do I calculate battery runtime?
Runtime in hours = capacity in Ah × voltage × usable share × efficiency ÷ load in watts. A 100 Ah, 12 V battery holds 1,200 Wh. Using 80 percent of it with a 90 percent efficient inverter leaves 864 Wh, which runs a 100 W load for 8.64 hours, or 8 h 38 min. The battery supplies about 8.3 A to the inverter.
What is the formula for battery runtime?
Runtime in hours = capacity in Ah × voltage × usable share × efficiency ÷ load in watts. The battery runtime calculator applies it exactly and rounds only the displayed answer to six significant figures.
What is Peukert's effect?
Lead-acid batteries deliver less total energy when discharged quickly. A heavy load gives a shorter runtime than the simple sum suggests.
How do I convert mAh to Wh?
Multiply the Ah by the voltage. For example 5 Ah at 3.7 V is 18.5 Wh. The tool also accepts milliamp-hours.
What load should I enter?
The average power in watts. For a device in amps, multiply by the battery voltage.
Is this battery runtime calculator free and private?
Yes. It is free, needs no account and runs in your browser, so the numbers you type are not uploaded or stored.