RuntimeCalculator
Battery Runtime Calculator
Hours from amp-hours or watt-hours, with discharge, inverter, and temperature included.
How it works
Wh rated = Ah × V (Skip that step if you type watt-hours.) usable Wh = Wh rated × depth of discharge × temperature factor × inverter efficiency hours = usable Wh / average watts average watts = sum of (watts × duty) for each enabled row duty is the share of time the device is actually drawing power (0 to 1) DC amps, no inverter: usable Ah = Ah × depth of discharge × temperature factor hours = usable Ah / amps Size a battery: usable Wh needed = watts × target hours rated Wh = usable Wh / (depth of discharge × temperature factor × efficiency) rated Ah = rated Wh / volts Conservative planning hours = hours × 0.85 That 0.85 factor is an editorial margin, not a second physical model.
The preset is a 100 Ah, 12 V LiFePO4 battery, 90% depth of discharge, a 90% inverter, 77°F, and a steady 60 W load. That is 972 Wh usable and about 16.2 hours. Change any input and the result updates immediately. The link in the address bar updates too, so you can copy a scenario without an account.
Average watts are one load, or each row's watts times its duty cycle. Duty cycle is the share of time a device is actually drawing power. A compressor fridge listed at 50 W that runs about 40% of the time counts as 20 W average. The list holds up to 8 rows. Scenario templates fill example watts. They are examples. Edit them.
If you type amps and choose DC only, the math switches to amp-hours: usable amp-hours divided by amps. With an inverter in the path, use watts so inverter loss still reduces runtime. Battery-side current is watts divided by voltage and efficiency. That current divided by amp-hours is the C-rate. Lead-acid above about 0.2C gets a warning because linear amp-hour math is optimistic there. Above about 0.5C the warning is stronger. LiFePO4 warns above 1C. The warning does not change the hour number. The hours stay on the linear model.
A second line multiplies the hours by 0.85. That conservative figure is an editorial planning margin, not a measurement of age or a second physical model. Real packs still vary with the maker's BMS, how old the battery is, and the temperature of the cells rather than the air.
Choose "Size a battery" when you know the load and the hours you want. The same factors run in reverse and return rated watt-hours and amp-hours. The cards under that result are example searches and brand catalogs. They are not hands-on tests, and this site does not store a price. This page does not estimate phone or laptop milliamp-hour life, and it does not estimate how many years a battery will last on the calendar.
Worked examples
100 Ah LiFePO4 at 60 W
12 V, 90% depth of discharge, 90% inverter, 77°F, one steady load. This is the default on this page.
- Rated watt-hours = 100 Ah × 12 V = 1,200 Wh.
- Usable watt-hours = 1,200 × 0.90 depth of discharge × 1.00 temperature × 0.90 inverter = 972 Wh.
- Hours = 972 Wh / 60 W = 16.2 h.
- Conservative planning figure = 13.77 h, which is the estimate times 0.85.
Same battery, fridge plus a steady load
A 50 W fridge at duty 0.40 plus a 60 W device at duty 1.00. Nothing else changes.
- Average watts = 50 × 0.40 + 60 × 1.00 = 80 W.
- Usable watt-hours stay 972 Wh.
- Hours = 972 / 80 = 12.15 h.
Questions
How do you calculate battery runtime?
Multiply amp-hours by volts to get rated watt-hours. Multiply by depth of discharge, the temperature factor, and inverter efficiency to get usable watt-hours. Divide usable watt-hours by the average load in watts. The methodology page writes the same formula.
How long will a 100 Ah 12 V LiFePO4 battery last?
With the defaults on this page (90% depth of discharge, 90% inverter efficiency, 77°F), usable energy is 972 Wh. A steady 60 W load then runs about 16.2 hours. Change the watts, the duty cycle, or the temperature and the result changes. This is an editorial estimate, not a measurement of a specific battery.
Does depth of discharge matter?
Yes. Depth of discharge is the fraction of nameplate capacity you plan to use. The LiFePO4 preset uses 90%. AGM, flooded, and gel presets use 50%. You can move the slider. A deeper discharge adds hours in the estimate and usually costs cycle life on the real battery.
Should I include inverter efficiency?
Include it for AC loads. The default is 90%, so the battery must supply more than the AC watts. Choose DC only for a load wired straight to the battery. That sets efficiency to 100% and hides the inverter slider.
Why do lead-acid calculators look more optimistic at high current?
A linear amp-hour model assumes the nameplate amp-hours are available at any current. Lead-acid batteries deliver less capacity as current rises. This site keeps the linear result and shows a warning above about 0.2C on AGM, flooded, and gel, with a stronger warning above about 0.5C. The warning is an editorial flag. The pages do not apply a Peukert curve unless you are reading the optional formula on the methodology page.
What is the difference between a battery runtime calculator and a battery life calculator on this site?
Both phrases land here, and both mean the same tool: how long a battery can power a load. This site does not estimate phone or laptop milliamp-hour battery life, and it does not estimate calendar years before a battery wears out. If you have a house battery, a power station, or a UPS bank, you are in the right place.
Related calculators
- Amp hour calculator. Convert Ah and Wh, then estimate runtime.
- UPS runtime calculator. Router, modem, laptop, and monitor with duty cycle.
- Battery capacity calculator. Size the amp-hours and watt-hours a load needs.
These figures are editorial estimates. The formula, chemistry defaults, and temperature table are written out on themethodology page.
Updated October 5, 2026