Estimate how long a battery, portable power station, UPS, or stored-energy source can operate a user-entered load. Work from Wh or from Ah plus voltage, combine devices, or solve for required capacity or a maximum average power budget.
Battery energy is normalized to watt-hours and load power to watts. Usable capacity is applied first, then the separately entered system efficiency. Deliverable watt-hours divided by average watts gives estimated hours.
The result is an estimate from your inputs, not an electrical safety rating or a prediction of battery chemistry behavior.
runtime hours = battery Wh × usable fraction × efficiency fraction ÷ average load Wbattery Wh = Ah × nominal voltage; mAh ÷ 1,000 = Ahcombined W = Σ(power W × quantity × duty-cycle fraction) + system idle Wrequired nominal Wh = load W × target hours ÷ (usable fraction × efficiency fraction)maximum average W = deliverable Wh ÷ target hours1,000 Wh at a 100 W load with 100% usable capacity and efficiency lasts an estimated 10 hours.
1,000 Wh at 90% usable and 85% efficiency provides 765 Wh, or 7.65 hours at 100 W (7 hr 39 min).
100 Ah at 12 V equals 1,200 Wh and lasts 10 hours at 120 W before losses.
20,000 mAh at 5 V equals 20 Ah × 5 V = 100 Wh, or 10 hours at 10 W before losses.
A 60 W laptop, 30 W fan, and 10 W light total 100 W and run for 10 hours from 1,000 deliverable Wh.
A 100 W load for 10 hours at 80% usable and 90% efficiency requires about 1,388.89 Wh (1.389 kWh).
A 1,000 Wh battery at 90% usable and 90% efficiency provides an average power budget of 101.25 W for 8 hours.
When the manufacturer supplies watt-hours, select Energy capacity. Voltage is not needed.
For variable loads, enter measured or representative average watts rather than peak watts.
Use usable capacity for reserved or unavailable stored energy and system efficiency for inverter, conversion, or wiring losses.
Watt-hours measure energy. Amp-hours measure charge, so voltage is required to convert Ah or mAh to Wh. Comparing mAh across different voltages is not an energy comparison.
Usable capacity describes how much nominal stored energy you intend to use. System efficiency describes how much usable energy reaches the load. Both default to 100% so the calculator introduces no hidden assumption.
Included devices are converted to watts, multiplied by quantity, and adjusted by the user-entered duty cycle. An optional explicit system or inverter idle load is then added.
Required-capacity mode solves for nominal Wh and optionally Ah at an entered voltage. Power-budget mode estimates the maximum average load for a target runtime; it is not an inverter or wiring safety limit.
Use the current rated or measured capacity and an average load. Manufacturer reserve capacity, inverter shutdown thresholds, and rapidly changing UPS loads can make actual runtime different.
Battery chemistry, age, temperature, voltage behavior, battery-management limits, discharge rate, inverter losses, load variation, and manufacturer reserve capacity affect real runtime. Lead-acid capacity may also vary with discharge rate through the Peukert effect, which is not applied here.
Multiply battery Wh by usable capacity and efficiency fractions, then divide by average load watts.
The ideal estimate is 10 hours at 100% usable capacity and efficiency.
Multiply amp-hours by nominal voltage.
Divide mAh by 1,000 to get Ah, then multiply by voltage.
Amp-hours measure charge rather than energy; voltage is required to calculate watt-hours.
It is the percentage of nominal energy you expect to make available after any reserve or manufacturer limit.
Enter a measured or documented estimate for your system. The calculator defaults to 100% and does not guess losses.
Lower efficiency reduces deliverable energy and therefore runtime.
Add included device watts after quantity and duty-cycle adjustments, then divide deliverable energy by that combined average load.
Required-capacity mode divides load watts times target hours by usable and efficiency fractions.
Power-budget mode divides deliverable Wh by target hours to estimate maximum average load.
Actual capacity, temperature, aging, conversion losses, changing loads, shutdown limits, and discharge behavior may differ from your inputs.
Yes, but this calculator does not infer chemistry-specific behavior.
Yes. Enter current estimated capacity or lower usable capacity; no hidden degradation factor is applied.
No. Peukert behavior is chemistry- and discharge-dependent and requires additional parameters.
Yes. Use its Wh capacity and your average load, then enter any efficiency or usable-capacity adjustment you choose.
Yes as an energy estimate, but actual UPS runtime and output limits must be verified with the manufacturer.
No. It estimates energy and runtime only. Follow manufacturer output limits and qualified guidance for wiring, fuses, breakers, current, inverters, and battery safety.