Methodology
Every calculator on this site calls the same functions. A result is an editorial estimate: it is the published formula applied to the inputs on the page, including defaults you can change. It is not a lab measurement of a specific battery, and it is not a promise that your gear will run for that time.
Formula
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.
Inverter efficiency is 0.90 for AC loads unless you move the slider. DC only sets efficiency to 1.00. Power-station mode treats the watt-hour number you type as rated watt-hours and starts depth of discharge at 1.00, because many labels are already an energy figure. Lower the slider if you want a margin. The label is still the maker's number, not a measurement we made.
Chemistry defaults
Changing chemistry updates the depth-of-discharge slider to that default. You can move the slider afterward.
| Chemistry | Default depth of discharge | Note |
|---|---|---|
| LiFePO4 | 0.90 | Recommended default. |
| Li-ion (NMC and similar) | 0.80 | Lithium that is not LiFePO4. |
| AGM | 0.50 | Conservative deep-cycle default. |
| Flooded lead-acid | 0.50 | Same planning default as AGM. |
| Gel | 0.50 | Grouped with other lead-acid types. |
Temperature factor
The input is ambient temperature in °F, default 77°F (25°C). The factor is an editorial step table, not a lab curve. 68°F and warmer uses 1.00. A temperature on a boundary uses the band that names that boundary, so 32°F is in the 32 to 49 band.
| Temperature | LiFePO4 and Li-ion | AGM, flooded, gel |
|---|---|---|
| 68°F and warmer | 1.00 | 1.00 |
| 50 to 67°F | 0.95 | 0.90 |
| 32 to 49°F | 0.85 | 0.75 |
| 14 to 31°F | 0.70 | 0.60 |
| Colder than 14°F | 0.55 | 0.45 |
High current
C-rate is battery-side current divided by rated amp-hours. The pages keep the linear hour result and add a warning. Lead-acid above about 0.2C: the linear model is optimistic. Above about 0.5C the warning is stronger. LiFePO4 and other lithium above 1C: many packs allow a high rate, but they are not unlimited. Check the continuous-current rating.
A simplified Peukert expression exists in the code and is covered by tests: t = H × (C / (I × H))^n, with H = 20 hours as an editorial rate assumption. The exponent is 1.3 for lead-acid and 1.05 for lithium. It is off unless a caller sets the flag. These pages do not turn it on, so the published hours match the linear model.
What this does not model
- Series and parallel wiring of a bank.
- Solar recharge or state of charge over several days.
- Calendar aging, except the optional 0.85 planning line.
- Startup surge, inverter idle draw, or cable loss.
- Phone or laptop milliamp-hour "battery life."
- Whether a medical device is appropriate. CPAP rows are power estimates only.
Use the battery runtime calculator, the amp hour calculator, the UPS runtime calculator, or the battery capacity calculator.
Updated October 5, 2026