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Batteries

The battery runtime formula

Usable energy divided by load. The formula is trivial; getting the two inputs right is where runtime estimates fail.

Published 23 August 2026 · Published by SnapEnergyLab. Methodology and default assumptions are documented separately.

A wall-mounted home battery and inverter in a daylit utility room, powering a refrigerator and a lamp during an outage

Quick answer

Runtime is usable energy divided by average load. The formula is trivial. Getting the two inputs right is where every optimistic runtime estimate falls apart.

“Usable” is not the number on the box: it is nominal capacity reduced by depth of discharge and then by inverter conversion. “Load” is not the sum of the nameplate ratings: it is the average draw over the period, which for cycling appliances is far lower.

How to read the numbersYour inputSite defaultCalculated result
Usable AC energy
8.28 kWh
Calculated result
Average load
500 W
Your input
Runtime
16 h 34 min
Calculated result

The formula, term by term

  1. usable AC energy (kWh) = nominal capacity (kWh) × DoD × inverter efficiency
  2. runtime (h) = usable AC energy (kWh) ÷ average load (kW)
  4. example: 10 kWh × 0.90 × 0.92 = 8.28 kWh
  5. 8.28 ÷ 0.50 kW = 16.56 h

Every term after the first reduces the answer. There is no term in this formula that makes runtime longer than nominal capacity divided by load.

Nominal capacity is the datasheet figure, in kWh. If your battery is specified in amp-hours, convert it first — amp-hours are meaningless without the voltage.

Depth of discharge is the share you are allowed to use, typically 0.5 for lead-acid and 0.9–1.0 for LFP home batteries.

Inverter efficiency converts DC in the battery to AC at the socket, usually 0.90–0.95 at moderate load. DC loads fed directly, such as 12 V lighting, skip this term.

Average load is the mean power over the whole discharge, including everything that switches on and off during it.

Runtime against load

The relationship is a hyperbola, not a straight line, and that shape is the practically important part: halving the load doubles the runtime, but adding one 1,500 W appliance to a 300 W baseline cuts runtime by more than 80%. Backup planning is dominated by whether you run the heavy thing at all.

Runtime of a 10 kWh nominal battery against continuous AC load0 h25 h50 h75 h500 W1000 W1500 W2000 WContinuous AC load (W)Runtime (hours)
10 kWh nominal capacity, 90% depth of discharge, 92% inverter efficiency — all site defaults — giving 8.28 kWh delivered to AC loads. Assumes a constant load, no temperature derating and no inverter standby beyond the efficiency figure.
Show the plotted values
Continuous loadRuntime (hours)
100 W82.8 h
200 W41.4 h
300 W27.6 h
500 W16.56 h
750 W11.04 h
1000 W8.28 h
1500 W5.52 h
2000 W4.14 h
Runtime by load for a 10 kWh nominal battery delivering 8.28 kWh AC
LoadExampleRuntime
100 WRouter, lighting, phone charging82 h 48 min
200 WAbove plus a laptop and a modest TV41 h 24 min
300 WFridge and freezer averaged over their cycles, plus lighting27 h 36 min
500 WTypical whole-house backup essentials16 h 34 min
750 WEssentials plus a circulation pump11 h 02 min
1000 WEssentials plus a small well pump running8 h 17 min
1500 WA space heater on its own5 h 31 min
2000 WA kettle or a hob ring4 h 08 min

Worked example: fridge, freezer and lighting overnight

  1. Fridge, 150 W nameplate at 35% duty52.5 W averageCalculated result
  2. Freezer, 120 W nameplate at 40% duty48 W averageCalculated result
  3. LED lighting and router60 WIllustrative model
  4. Average load160.5 W = 0.1605 kWCalculated result
  5. Usable AC energy10 × 0.90 × 0.92 = 8.28 kWhCalculated result
  6. Runtime8.28 ÷ 0.1605 = 51.6 hCalculated result
Runtime on essentials only≈ 52 hours

What makes real runtime shorter than calculated

Temperature. A pack in an unheated garage delivers measurably less than its rated capacity.

Age. Capacity fade of a few percent a year is typical for lithium cells, and most manufacturer warranties guarantee only 70–80% of original capacity after ten years — so an older pack's nameplate figure is optimistic by an amount that grows with age.

Inverter standby. An inverter idling draws 10–50 W whether or not anything is plugged in. Over a 45-hour discharge that alone can be 1–2 kWh.

Load creep. Backup discharges rarely stay at the planned load, because people boil kettles.

Run this with your own capacity and load

Enter capacity, DoD, inverter efficiency and load. The calculator shows delivered AC energy and runtime, and the backup builder lets you assemble a load list appliance by appliance.

Open the battery runtime calculator

Assumptions and limitations

  • Duty cycles used in the worked example are typical values, not measurements of your appliances.
  • The chart assumes a constant load. Real backup loads vary continuously, and the runtime you get is set by the average, which is hard to know in advance.
  • Temperature derating, capacity fade and inverter standby consumption are excluded from the chart and would all shorten the result.

Sources

  1. IEC 62040 — Uninterruptible power systems, performance and test requirements — International Electrotechnical Commission

    Supports: Runtime is specified against a stated load and stated conversion efficiency, not against nominal capacity alone.

  2. Battery test manual for stationary energy storage — U.S. Department of Energy / Idaho National Laboratory

    Supports: Deliverable energy depends on discharge rate and temperature as well as rated capacity.

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