Battery Runtime & Discharge Calculator

How to use this battery runtime calculator

1. Choose Battery Ah and voltage when you know an individual battery’s nominal voltage and amp-hour rating. Choose Total watt-hours when the complete bank or power station already gives one Wh rating.
2. Enter the capacity rating exactly once. In a series/parallel bank, open Realistic inputs and enter the number in series and the number of parallel strings.
3. Enter each load in watts, or use amps together with the load voltage. Choose direct DC or an AC inverter path.
4. For a duty-cycle load, enter its active draw, sleep draw, and percentage of time active. The calculator uses the weighted average.
5. Add only factors you can defend: planned depth of discharge, measured state of health, a datasheet temperature factor, inverter efficiency at the relevant load, and inverter idle draw.
6. Use Peukert mode only when the battery documentation supplies an exponent and capacity-rating period. It is off by default.
7. Read the energy steps, assumptions, and warnings with the result. Treat the number as an estimate, then compare it with the battery, BMS, inverter, wiring, protection, and measured-load limits.

For a field-by-field explanation, see how to collect battery runtime inputs. If the appliance label is ambiguous, start with measuring electrical load for runtime estimates.

What the result means

For an ideal constant load, runtime in hours is usable battery energy in watt-hours divided by battery-side power in watts. The calculator returns one reproducible estimate from the values you entered. It does not invent a low/base/high range.

The estimate is not a guarantee. Actual operation can stop sooner because of cutoff voltage, battery-management behavior, temperature, age, rate-dependent capacity, imbalance, voltage sag, transient or surge load, and inverter behavior. The exact battery and inverter documentation controls those limits.

Ah, Wh, and nominal voltage

Amp-hours measure electric charge. Watt-hours measure energy. A watt load needs an energy basis, so an Ah value alone is incomplete without voltage:

nominal Wh = nominal V × rated Ah

For identical batteries:

• series raises bank voltage: Vbank = Vunit × series count;
• parallel raises bank capacity: Ahbank = Ahunit × parallel count;
• total energy is Vunit × Ahunit × series × parallel.

Do not add a direct total-Wh rating to a second Ah-derived rating for the same bank. The calculator makes these capacity modes mutually exclusive.

AC and DC load paths

A direct DC load is added at its average watts. An AC load draws more from the battery than it delivers to the appliance because the inverter has conversion loss:

AC battery watts = AC output watts ÷ inverter efficiency

Inverter idle draw is different from conversion efficiency. The calculator adds idle watts once for the active inverter rather than once for every appliance row.

If a load is entered in amps:

watts = volts × amps

Use voltage at the load for this conversion. Do not use a 12 V assumption for a load whose actual operating voltage is different.

Duty-cycle loads

Loads such as pumps, radios, refrigerators, and controllers may alternate between active and sleep states. Their average is:

average W = active W × duty fraction + sleep W × (1 − duty fraction)

Measure over a representative interval whenever possible. A brief instantaneous reading can miss startup, cycling, or standby behavior.

Usable energy and the ideal model

The energy-balance model is:

usable Wh = nominal Wh × depth-of-discharge factor × health factor × temperature factor

runtime hours = usable Wh ÷ battery-side W

Quick mode leaves all three factors at 100%, which is an ideal calculation rather than a claim about a battery chemistry. Realistic mode keeps the factors visible and editable. BrowserPowerTools does not ship generic lithium, LiFePO4, AGM, lead-acid, or other chemistry defaults.

Optional Peukert calculation

Peukert behavior relates the capacity rating and constant discharge current. Using rated capacity Cr, rating period Hr, exponent n, and average current I:

Ir = Cr ÷ Hr

Cp = Irⁿ × Hr

full-to-cutoff time = Cp ÷ Iⁿ

The exponent is battery-specific. Victron’s technical documentation describes the calculation as an approximation and explains deriving an exponent from two capacity ratings. Do not choose an exponent merely from a chemistry name.

This calculator applies your depth-of-discharge, health, and temperature factors after the constant-current result as a plainly labeled analysis approximation. Do not use Peukert mode for a materially varying voltage/current profile when a product-specific discharge curve or time-step model is required.

Worked examples

1. Ideal 12 V battery and DC load

A 100 Ah, 12 V battery has 1,200 nominal Wh. With a user-planned 80% depth of discharge and a constant 100 W DC load:

1,200 Wh × 0.80 ÷ 100 W = 9.6 hours

This is an energy-balance example, not a promise that every 100 Ah battery supplies 9.6 hours.

2. The same load through an inverter

For a 100 W AC load, 90% inverter efficiency, and 5 W idle draw:

battery-side W = 100 ÷ 0.90 + 5 = 116.111 W

runtime = 960 Wh ÷ 116.111 W = 8.268 hours

The conversion loss applies to the AC load. Idle draw is then added separately.

3. Duty-cycle load

A device draws 100 W while active, 10 W asleep, and is active 25% of the time:

average W = 100 × 0.25 + 10 × 0.75 = 32.5 W

A 500 Wh ideal battery therefore gives 500 ÷ 32.5 = 15.385 hours before battery-specific corrections.

4. Series/parallel bank

Six identical 12 V, 100 Ah batteries arranged as two in series and three parallel strings form a 24 V, 300 Ah bank:

battery count = 2 × 3 = 6

energy = 24 × 300 = 7,200 Wh

A 600 W ideal load produces 7,200 ÷ 600 = 12 hours.

Common mistakes

• Treating mAh as Ah without dividing by 1,000.
• Entering Ah without nominal voltage for a watt load.
• Adding a direct Wh rating to the Ah-derived energy of the same battery.
• Assuming series increases Ah; series increases voltage for identical batteries.
• Applying inverter efficiency to direct DC loads.
• Omitting inverter idle draw or adding it once per appliance instead of once per inverter.
• Using peak watts as continuous average power, or ignoring cycling and standby draw.
• Treating nameplate Wh as fully usable without considering cutoff and the operating plan.
• Using charge efficiency as inverter efficiency or discharge usable fraction.
• Selecting a Peukert exponent from chemistry rather than the battery specification.
• Reporting more precision than the inputs and battery model support.

Troubleshooting

The tool says runtime is undefined

The total average load is zero. A zero denominator would produce an infinite mathematical value, which is not a useful runtime prediction, so the calculator withholds the result.

The result is much longer than observed runtime

Check measured average load, inverter idle draw, cutoff voltage, usable depth, battery condition, temperature, and capacity rating basis. Confirm that amps were paired with the correct voltage. At higher currents, use the exact product data and consider whether a product discharge curve or battery-specific Peukert input is appropriate.

The result changed when I selected AC

AC mode converts appliance watts into battery-side watts using your inverter efficiency and adds idle draw. Direct DC mode does not apply this loss.

Peukert mode will not run

It needs explicit bank voltage, bank Ah, rated hours, and an exponent between 1 and 1.5. Direct Wh without a bank voltage cannot establish current. Missing values are never filled from a generic chemistry preset.

Frequently asked questions

How long will a 100 Ah battery run a 100 W load?

Voltage matters. At 12 V, 100 Ah represents 1,200 nominal Wh, giving 12 ideal hours at 100 W. A planned 80% usable fraction gives 9.6 energy-balance hours before other losses or rate effects.

Should I enter Ah or Wh?

Use the value the complete battery or bank provides most directly. If you enter Ah, also enter nominal voltage. If a power station publishes total Wh, use direct-Wh mode and do not reconstruct and add another capacity.

What does C20 mean?

It means the stated capacity was measured over a 20-hour discharge basis under the product’s stated test conditions. It is not a universal runtime multiplier.

Does putting batteries in series increase runtime?

Series increases voltage, while parallel increases Ah. Runtime depends on total Wh and the load’s battery-side power. Compare complete energy and topology, not Ah alone.

What inverter efficiency should I enter?

Use the efficiency at the relevant load from the exact inverter documentation or a trustworthy measurement. Do not assume one universal value.

Can I use this for LiFePO4?

The energy-balance mode can use explicit ratings and factors for any battery when the model fits the task. The calculator does not infer LiFePO4 usable depth, cutoff, temperature behavior, or a Peukert exponent.

Why might actual runtime be shorter?

Possible reasons include cutoff, inverter losses and idle draw, temperature, battery condition, high discharge rate, cell imbalance, transient load, and rating conditions that differ from the project.

Is this a wiring or battery-safety calculator?

No. It does not select wire, fuse, BMS, charger, ventilation, enclosure, or protection, and it does not approve a battery installation.

Calculation boundary and verification

This page is a planning estimator. Confirm continuous and surge current, cutoff voltage, permissible depth of discharge, temperature range, wiring, overcurrent protection, charger, BMS, and inverter ratings separately. Use the current documents for the exact products, and seek qualified review for safety-critical or professional work.

The engine performs all calculations locally in the browser. No capacity, voltage, load, runtime, label, or project state is sent to a calculation server. CSV and JSON downloads are created locally and include the visible result, assumptions, source IDs, engine version and explicit setup metadata so the estimate can be audited later.

References and further reading

These sources define or explain the calculation. Source editions and model limits are part of the result—not decorative citations.

1. Battery capacity and Peukert exponent, Victron Energy; Battery capacity and Peukert exponent; accessed 2026-08-22. BAT-C01
2. Battery Life Calculator, DigiKey; accessed 2026-08-22. BAT-C02
3. DOE Explains...Batteries, U.S. Department of Energy; accessed 2026-08-22. BAT-C03
4. NIST Guide to the SI, National Institute of Standards and Technology; accessed 2026-08-22. BAT-C04