Battery Runtime Calculator

Runtime from usable energy, the Peukert correction for lead-acid, and capacity sizing

A battery’s nameplate amp-hours are not the energy you may spend: chemistry sets a safe depth of discharge, an inverter takes its cut, and lead-acid quietly shrinks at high currents. This calculator runs the honest chain: total energy (volts × amp-hours), usable energy after your depth of discharge and efficiency, and runtime against a load entered in watts or amps. An optional Peukert block applies the documented lead-acid rate correction, and a sizing mode reverses the arithmetic to the capacity a target runtime requires. Chemistry presets set sensible discharge defaults, every one of them editable, because the battery maker’s own limits outrank any table here.

Sizing mode reports the required figure here instead.
100 for a direct DC load; an inverter typically takes the figure to about 85.
Typically 1.1 to 1.3 for lead-acid, near 1.05 for lithium. With the load in amps, enables the rate-corrected runtime.
The period the Ah rating was measured over, usually 20 hours.

Disclaimer: This calculator gives estimates for general information only. It is not financial, medical or other professional advice. Battery systems for backup, medical or safety loads belong with the battery and equipment makers’ specifications and a qualified installer.

What this calculator does

The energy method prices everything in watt-hours, the only currency every part of the system shares: the battery stores volts × amp-hours of them, the discharge limit and the inverter ration them, and the load spends them at its watt rate. The Peukert block then handles the effect the energy method misses for lead-acid: capacity is rated at a gentle 20-hour draw, and pulling current faster shrinks the effective capacity by the documented exponent law, which the page applies openly with your exponent and rating period. Sizing mode divides a target runtime back through the same chain, so a requirement (“run 50 W for 8 hours”) becomes a purchasable capacity with the same assumptions attached.

How to use it

  • Depth of discharge is a lifespan decision. Lead-acid cycled to 50 percent lasts multiples of the cycles it survives at 80 percent; lithium chemistries tolerate deeper use. The presets encode the common practice, the field accepts the maker’s figure, and the FAQ explains why “usable” is doing work in the name.
  • Efficiency belongs to the conversion, not the battery. DC loads fed directly run at 100 in this model; anything through an inverter pays the inverter’s efficiency, and loads quoted in AC watts must be entered as the inverter sees them, which the worked sizing example demonstrates.
  • Peukert needs the load as a current. Enter the load in amps (or read the implied current in the results) and set the exponent from the battery maker’s data where published; the difference it makes at high draw is the difference between the brochure runtime and the real one.
  • Cold batteries are smaller batteries. Capacity falls with temperature, sharply for lead-acid near freezing; the figures here are room-temperature numbers, and winter backup sizing should carry margin the calculator cannot invent for you.

The formulas

  • Total energy = voltage × amp-hours. Usable energy = total × depth of discharge × efficiency. Runtime = usable energy / load watts (load amps × voltage first, where given in amps).
  • Peukert runtime = rating period × (rated capacity / (current × rating period)) ^ exponent, scaled by the discharge fraction: the documented lead-acid rate model.
  • Required capacity = load watts × target hours / (voltage × discharge × efficiency).

Worked examples

The caravan battery. 12 V, 100 Ah lead-acid, 50 W load, discharge limited to 50 percent, through an inverter at 85 percent: total energy 1,200 Wh, usable 510 Wh, runtime 10.20 hours. The nameplate alone (1,200 Wh / 50 W) promises 24 hours, a figure that would murder the battery and still not arrive, which is the entire argument for the two discount fields.

The Peukert correction. The same battery drawn at 10 A (120 W) with an exponent of 1.15 and a 20-hour rating: the simple energy model at full discharge predicts 10 hours; Peukert says 9.01 hours, and at the 50 percent discharge limit the honest figure is about 4.5 hours against the naive 5. The gap widens brutally with current and exponent, which is why high-draw lead-acid systems disappoint their owners on schedule.

Sizing backwards. To run 50 W for 8 hours on the caravan’s assumptions (12 V, 50 percent, 85 percent): required capacity = 400 / (12 × 0.5 × 0.85) = 78.43 Ah, so the 100 Ah battery is the purchasable answer, with its margin visible instead of accidental.

Reading your results

Read runtime as a range in disguise: battery age, temperature and the load’s real appetite all move it, and the honest planning figure sits below the printed one. Read the Peukert and energy results together: where they agree, the draw is gentle; where they split, the current is eating the battery’s rated capacity and a bigger bank or a lower draw is the fix. Read required capacity against stocked sizes and round up, because capacity is bought in steps and the next size up is the margin.

Limitations

  • Constant load assumed. Cycling loads (fridges, pumps) spend at their duty cycle, not their nameplate; multiply by the honest duty fraction before entering, or the runtime inherits the error.
  • Peukert is a lead-acid model. It is the documented chemistry model the brief permits; lithium behaviour is far flatter and the exponent near 1.05 reflects that, but no exponent turns this page into a battery management system.
  • Ageing and temperature are not modelled. A three-year-old battery at 5°C is not its datasheet; capacity testing, not calculation, settles real banks.
  • Voltage sag and cutoffs end runs early. Inverters disconnect at low voltage under load before the energy is truly spent, especially at high currents; treat runtimes at heavy draw as optimistic.

Frequently asked questions

Why can I not use all 100 amp-hours?

Because the rating counts charge the chemistry would rather keep: discharging lead-acid fully sulfates it toward an early death, and even lithium ages faster at the extremes of its range. Depth of discharge is the fraction the maker and practice allow you to spend routinely, and the runtime honest answer multiplies by it. Occasional deeper use is possible (the 100 percent preset exists), purchased with cycle life the calculator cannot refund.

What is the Peukert effect in plain terms?

The battery’s capacity is measured at a slow sip; drink fast and some of the charge becomes effectively unreachable, lost to internal resistance and chemistry that cannot keep up, recovering partly if the battery rests. Peukert’s law, published in 1897 and still the standard lead-acid correction, quantifies it with one exponent: at the rating current the effect vanishes, above it the effective capacity falls by the power law the calculator applies, and below it the battery can even exceed its rating slightly.

My load is in amps at 240 volts AC. What do I enter?

Convert to watts first (amps × volts = 240 × your amps) and enter watts, with the inverter’s efficiency in the efficiency field: a 2 A television is 480 W, which through an 85 percent inverter spends the battery like a 565 W DC load, a conversion the energy method handles exactly once the load is expressed in the shared currency of watts. Entering “2” in the amps field would describe a 24 W load at 12 V and understate the draw twentyfold.

How do I combine batteries in parallel or series?

Series adds voltage at the same amp-hours (two 12 V 100 Ah in series are a 24 V 100 Ah bank); parallel adds amp-hours at the same voltage (the same pair in parallel are 12 V 200 Ah). Energy is identical either way (2,400 Wh), which is why the energy method sizes banks without caring about the wiring: enter the bank’s resulting voltage and capacity and the calculation proceeds unchanged.

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Method: watt-hour energy chain with explicit discharge and efficiency fractions, the Peukert lead-acid rate correction, and the inverse sizing identity. Last reviewed: 14 October 2026.