How the Battery Runtime Calculator works

The Battery Runtime Calculator first converts the battery rating into nominal watt-hours. An amp-hour rating is multiplied by battery voltage, while milliamp-hours are divided by 1,000 before the same conversion. A watt-hour rating can be used directly. The tool then reduces nominal energy by the chosen usable-capacity percentage, system efficiency and safety reserve before dividing by the load.

Electrical calculations use volts, amperes, watts and watt-hours. The NIST guide to electric-current units explains the ampere and related SI quantities.

Battery runtime formulas

Nominal energy: Wh = Ah × V. For mAh, Ah = mAh ÷ 1,000. Usable delivered energy: nominal Wh × usable capacity × efficiency × (1 − reserve). Runtime: usable delivered Wh ÷ load W. When load is entered in amperes, equivalent load power is A × V.

Worked Battery Runtime Calculator example

A 12-volt, 100-Ah lead-acid battery stores 1,200 nominal watt-hours. With 50% usable capacity, 90% system efficiency and a 10% reserve, about 486 watt-hours remain available to the load. A steady 100-watt load therefore has an estimated runtime of 4.86 hours, or approximately 4 hours 52 minutes.

Battery type and usable capacity

Rated capacity is not always the energy that should be removed in normal service. The preset percentages are general planning defaults, not manufacturer limits. Lead-acid batteries are often modeled with a shallower discharge than lithium batteries. Select Custom when a manufacturer specifies a different recommended usable capacity or depth of discharge.

Efficiency, reserve and real-world load

An inverter, converter, wiring and control electronics consume energy. Efficiency accounts for these losses, while the reserve keeps part of the modeled battery energy unused. Loads may also cycle or surge. For a refrigerator, pump or motor, use a realistic average power over time rather than only the nameplate peak or starting value.

Accuracy and limitations

The Battery Runtime Calculator is a steady-load energy model. Real batteries may provide less rated capacity at high discharge rates, low temperatures or advanced age. Lead-acid capacity is especially sensitive to discharge rate, often described by Peukert behavior. The tool does not model voltage sag, cell balancing, battery-management cutoffs, inverter idle consumption or changing load profiles. Verify critical backup, marine, vehicle, solar and medical-power plans with manufacturer data and qualified guidance.

Frequently asked questions

Should I enter Ah, mAh or Wh?

Use the unit printed on the battery. Voltage is required to convert Ah or mAh into watt-hours. When the label already gives Wh, the calculator uses that energy rating directly.

Why is calculated runtime shorter than capacity divided by load?

The estimate applies usable-capacity limits, conversion efficiency and the selected reserve. These adjustments prevent the model from treating every rated watt-hour as deliverable energy.

Can I enter a load in amperes?

Yes. Select Current and enter amperes. The calculator converts current to watts using the battery voltage.

Does the result include inverter losses?

Yes, when you enter an appropriate system-efficiency percentage. Consult the inverter efficiency curve when available because efficiency can change with load.

Is this suitable for solar batteries?

It can estimate battery-only runtime between charging periods. It does not add incoming solar production, so a full solar energy model may produce a different result.