Battery Runtime Calculator

Estimate how long a battery can run a device based on battery voltage, battery capacity, load watts, battery type, and inverter efficiency.

Enter Your Numbers

Battery bank voltage (V)

Examples: 12V, 24V, 48V.

Battery bank capacity (Ah)

Enter the total amp-hours of the battery bank.

Device load (watts)

Use the running wattage, not just startup surge.

Battery type

Custom usable battery %

Only use a custom value if you know what is realistic for your battery.

My device runs through an inverter

Leave this checked for most AC appliances. Uncheck it for direct DC loads.

Inverter efficiency (%)

Many real-world inverters are around 85% to 92% efficient.

Planning buffer (%)

Use a buffer to avoid overestimating runtime.

Your Results

Estimated runtime

—

Enter values and calculate.

Usable energy

—

Nominal energy: —

Estimated battery current

—

Approximate DC current drawn from the battery bank.

Battery settings used

—

Inverter efficiency: —

This calculator gives a planning estimate, not a guarantee. Real runtime is usually lower because of battery age, temperature, inverter losses, voltage cutoff, load variation, and startup surge.

How to Use This Battery Runtime Calculator

Enter your battery bank voltage, the total amp-hours of the battery bank, and the running wattage of the device. Then choose the battery type, decide whether the load runs through an inverter, and click Calculate Runtime.

If your setup uses multiple batteries, enter the final battery bank numbers:

Two 12V 100Ah batteries in parallel = 12V, 200Ah
Two 12V 100Ah batteries in series = 24V, 100Ah

Formula Used

Nominal watt-hours = Battery volts × Battery amp-hours
Usable watt-hours = Nominal watt-hours × Usable battery %
Buffered watt-hours = Usable watt-hours × (1 − Planning buffer %)
Delivered watt-hours = Buffered watt-hours × Inverter efficiency (if used)
Estimated runtime (hours) = Delivered watt-hours ÷ Load watts

Example Calculation

A 12V 100Ah LiFePO4 battery has about 1,200 watt-hours of nominal energy.

If you assume 90% usable capacity, use a 10% planning buffer, and run a 60W device through a 90% efficient inverter, the usable delivered energy is about 875 watt-hours.

875 Wh ÷ 60 W = about 14.6 hours of estimated runtime.

Why Real Runtime Is Often Lower

Battery age and condition reduce available capacity.
Cold weather can reduce battery performance.
Inverter efficiency changes with load.
Appliances may cycle on and off instead of drawing constant power.
Some devices have startup surge that this calculator does not model.
Lead-acid batteries should usually not be drained too deeply.

Important Safety Note

Use this as a planning tool only. Do not use it as permission to overload a battery, inverter, charger, wire, extension cord, or power system. Check the manufacturer ratings for your battery, inverter, fuse, and wiring before relying on any result.