RuntimeCalculator

Amp Hour Calculator

Watt-hours equal amp-hours times volts. Runtime uses what is left after real losses.

Advanced

Recommended default. Leaving a little capacity unused is easier on cycle life.

Real usable capacity still depends on the maker's BMS, age, and the temperature of the cells.

Template watts are examples. CPAP rows are power estimates only, not a medical clearance. Trolling-motor rows are estimates too: follow local water rules.

Editorial estimate

1,200 Wh nameplate, 100 Ah at 12 V

Runtime

8.1

8 h 6 min

1,200 Wh nameplate × 0.90 depth of discharge × 1.00 temperature × 0.90 efficiency = 972 Wh usable.

Usable energy
972 Wh
Average load
120 W
Battery current
11.11 A
C-rate
0.11
Conservative planning figure
6.89 h (× 0.85, editorial)
Depth of discharge
90%
Efficiency
90%
Temperature factor
1.00 at 77°F (25°C). Editorial estimate, not a lab curve.

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.

Amp-hours are charge. Watt-hours are energy. The bridge is voltage: Wh = Ah × V, and Ah = Wh / V. A 100 Ah battery at 12 V stores 1,200 Wh on the nameplate. The same 100 Ah at 24 V stores twice that energy. Comparing amp-hours across different voltages, without converting, will mislead you.

Nameplate watt-hours are not the same as usable watt-hours. Depth of discharge, cold temperature, and inverter efficiency all come off the top. At the defaults here (90% discharge, 90% inverter, 77°F) that 1,200 Wh nameplate becomes 972 Wh usable. A 120 W load then runs about 8.1 hours. The big nameplate line on this page is the conversion. The hours under it are the runtime.

Power-station mode asks for watt-hours directly. The example buttons (256, 512, 1024, and 2048 Wh) are capacity tiers, not model numbers. Switching to that mode sets depth of discharge to 100% so the label is used as rated energy. Lower the slider if you think the usable figure is below the label. Different makers do not measure that label the same way. This page will not claim that a named power station runs for a specific number of hours.

Milliamp-hours are amp-hours divided by 1,000. That conversion is arithmetic, not a phone-battery model. This calculator does not estimate screen-on time, and it does not take a laptop's milliamp-hour label and turn it into a workday. For a house battery that was printed in mAh, divide by 1,000, type the pack voltage, and continue here. If you already know the load in amps on a DC line with no inverter, switch the load unit to amps and choose DC only.

Worked examples

100 Ah at 12 V, then a 120 W load

LiFePO4, 90% depth of discharge, 90% inverter, 77°F. The default load on this page is 120 W so the conversion and the runtime are both easy to check.

  1. Nameplate watt-hours = 100 × 12 = 1,200 Wh.
  2. Equivalent amp-hours at 24 V would be 50 Ah for the same energy.
  3. Usable watt-hours = 1,200 × 0.90 × 1.00 × 0.90 = 972 Wh.
  4. Hours at 120 W = 972 / 120 = 8.1 h.

512 Wh example tier at 100 W

Power-station mode, depth of discharge 100%, inverter 90%, 77°F. The tier is an example capacity, not a product test.

  1. Rated watt-hours = 512 Wh, typed directly.
  2. Usable watt-hours = 512 × 1.00 × 1.00 × 0.90 = 460.8 Wh.
  3. Hours at 100 W = 4.61 h.

Questions

How do you convert amp-hours to watt-hours?

Multiply amp-hours by the battery voltage. 100 Ah at 12 V is 1,200 Wh on the nameplate. That product is energy, before depth of discharge and inverter loss.

How do you convert watt-hours to amp-hours?

Divide watt-hours by voltage. A 1,200 Wh pack at 12 V is 100 Ah. At 24 V the same 1,200 Wh is 50 Ah. Amp-hours are not comparable across voltages until you convert to watt-hours.

Are milliamp-hours the same as amp-hours?

1,000 mAh equals 1 Ah. Phone and laptop batteries are often labeled in mAh at a cell voltage near 3.6 V or 3.7 V. This calculator is for storage batteries and power stations, not for predicting handset screen-on time.

Does a higher voltage battery last longer?

Only if amp-hours stay the same, because energy is amp-hours times volts. A 100 Ah 24 V battery stores twice the watt-hours of a 100 Ah 12 V battery, so a watt load runs longer. A load specified in amps at the new voltage is a different load.

Why is runtime shorter than watt-hours divided by watts?

Watt-hours divided by watts ignores depth of discharge, cold temperature, and inverter loss. Those factors are multiplied into usable watt-hours here. A 1,200 Wh nameplate at 90% discharge and 90% inverter efficiency is 972 Wh usable at 77°F, not 1,200 Wh.

These figures are editorial estimates. The formula, chemistry defaults, and temperature table are written out on themethodology page.

Updated October 5, 2026