Battery Runtime Calculator

Estimate how long a battery will power a load from its amp-hours, voltage, usable depth of discharge and the load in watts. The result is the runtime in hours plus the usable energy in watt-hours.

Results are estimates for planning and education, based on your inputs and standard engineering values (AWG resistance, NEC ampacity, resistivity). Electrical work can be dangerous and is governed by the NEC and your local code — verify all sizing with a licensed electrician and your authority having jurisdiction (AHJ). Not a substitute for professional design.

Calculator

Estimated runtime8.0 h
Usable energy960 Wh

Formula

Runtime (h) = (Ah × V × DoD) / load (W). Usable energy (Wh) = Ah × V × DoD. Depth of discharge (DoD) is the fraction of the rated capacity you draw before recharging — roughly 0.8 for lithium (LiFePO4) and 0.5 for lead-acid.

Worked example

A 100 Ah 12 V battery at 80% usable depth of discharge feeding a 120 W load: usable energy = 100 × 12 × 0.8 = 960 Wh; runtime = 960 / 120 = 8.0 hours.

Runtime is capacity divided by draw, but with a catch: you can only use part of a battery's rated amp-hours. This tool applies the usable depth of discharge for your chemistry — deep for lithium, roughly half for lead-acid — so the runtime reflects what you can actually take out without harming the pack. It is the quickest way to judge whether the fridge and lights will last the night before you commit to a battery size. Real runtime also shrinks in the cold and at high discharge rates, so keep a margin, and remember that adding loads shortens the figure proportionally: the always-on draws matter as much here as the occasional big ones.

Frequently asked questions

How long will my battery last running a given load?
Multiply the amp-hours by the voltage to get the rated watt-hours, multiply by the usable depth of discharge, then divide by the load in watts. A 100 Ah 12 V battery holds 1,200 Wh rated, of which about 960 Wh is usable at an 80% lithium depth of discharge; powering a 120 W load that gives 8.0 hours. Real runtime is a little shorter because inverters, wiring and high discharge rates all waste a few percent, so treat the figure as an optimistic estimate.
What depth of discharge should I enter?
Depth of discharge is how deeply you cycle the battery before recharging. Lithium (LiFePO4) tolerates about 0.8–0.9 with little impact on lifespan, so 0.8 is a safe planning value. Lead-acid, AGM and gel last far longer if you only use about half their capacity, so use 0.5. Pushing lead-acid to 0.8 will run the load longer on paper but dramatically shortens battery life. Match the DoD to your chemistry to get a runtime you can actually repeat.
How do I find the load in watts?
If a device is rated in watts, use that directly. If it is rated in amps at a voltage, watts = volts × amps; the Appliance Amp Draw tool does the reverse when you only know watts. For an inverter running AC gear, add about 10% for inverter losses, since the battery must supply both the load and the conversion overhead. Add up every device that runs at the same time to get the total load this calculator expects.
Does a higher load drain the battery faster than linearly?
For lithium, runtime is close to linear: double the load and the time roughly halves. Lead-acid suffers from the Peukert effect — at high discharge currents the usable capacity shrinks, so a heavy load drains it faster than a simple division predicts. This tool uses the straightforward energy-balance method, which is accurate for lithium and slightly optimistic for lead-acid under heavy loads. Size in a margin if you regularly pull big currents from a lead-acid bank.
How is amp-hour runtime different from watt-hour runtime?
Amp-hours only describe capacity at a given voltage, so 100 Ah means very different energy at 12 V (1,200 Wh) versus 24 V (2,400 Wh). Working in watt-hours — amp-hours times voltage — lets you compare banks of different voltages fairly and divide cleanly by a load in watts. That is why this calculator converts to usable watt-hours first and then divides by the load, rather than dividing amp-hours by amps.
How long will a 100Ah battery run a fridge or a 12V appliance?
It depends on the appliance’s average draw, not its peak. A 12 V compressor fridge might average 40–50 W once you account for the compressor cycling on and off, so a 100 Ah lithium battery (960 Wh usable) could run it for roughly 19–24 hours. A 60 W fan runs about 16 hours; a 600 W microwave only minutes. Enter the device’s real average wattage to get its figure, and remember a fridge’s nameplate watts overstate the steady draw because it does not run continuously.
Should I add a margin to the calculated runtime?
Yes. The energy-balance result is an optimistic ceiling: inverter conversion, wiring resistance, cold temperatures and battery age all chip away at it, and lead-acid loses extra capacity under heavy load. Knocking 10–20% off the calculated hours gives a figure you can rely on in the field. If the runtime is critical — medical equipment, for example — design with a larger margin and a backup charging source rather than running the bank to the edge of its usable depth.

Source: Usable energy (Ah × V × DoD) divided by the load in watts · All sources

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