How battery runtime is calculated
Battery runtime is estimated by converting the battery capacity to watt-hours, reducing that energy by the selected usable-capacity and efficiency percentages, and then dividing the remaining watt-hours by the load in watts.
Convert amp-hours to watt-hours
If battery capacity is provided in amp-hours, voltage is needed to convert the capacity to energy.
For milliamp-hours, divide mAh by 1,000 before multiplying by voltage.
Battery runtime example
A 100 Ah, 12 V battery has a nominal energy capacity of 1,200 Wh. If 80% of that capacity is usable, 960 Wh remains. Applying 90% system efficiency leaves 864 Wh available to the load.
With a constant 100 W load, the estimated runtime is 8.64 hours, which is approximately 8 hours and 38 minutes.
What does usable capacity mean?
Nominal battery capacity does not necessarily mean that every watt-hour should or can be used in a real application. The usable capacity field lets you decide how much of the rated energy to include in the estimate.
The appropriate usable percentage depends on the battery, application, operating limits and manufacturer guidance. The calculator does not choose a depth of discharge for you.
What does system efficiency mean?
Power conversion equipment can consume some battery energy before it reaches the load. Inverters, DC-DC converters, wiring and other equipment may introduce losses.
The efficiency field reduces the available battery energy by the percentage you specify. For example, 90% efficiency means 90% of the energy remaining after the usable-capacity adjustment is treated as available to the load.
Using a load in amps
When load current is entered in amps, the calculator uses battery voltage to estimate DC load power.
If you only need to convert current and voltage to power, use the Amps to Watts Calculator.
Why real battery runtime can differ
This is an energy-based estimate, not a battery discharge model. Real runtime can differ because battery voltage changes during discharge, loads may vary over time, and usable capacity can be affected by temperature, battery condition, discharge rate, battery chemistry and equipment behavior.
Use the battery and equipment manufacturer specifications for operating limits, discharge ratings, minimum voltage, usable capacity and safety requirements. This calculator provides an approximate planning estimate.
Ah, mAh and Wh capacity
Amp-hours and milliamp-hours describe electric charge, while watt-hours describe energy. Batteries with the same Ah rating can contain different amounts of energy when their voltages differ.
If the battery is already rated in watt-hours and the load is entered directly in watts, voltage is not required for the basic runtime calculation.
Frequently asked questions
How long will a 100 Ah battery run a 100 W load?
At 12 V, the battery contains 1,200 Wh nominally. With the calculator's default 80% usable capacity and 90% efficiency, the estimate is 8.64 hours.
Can I enter battery capacity in watt-hours?
Yes. Select Wh as the capacity unit. If the load is also entered in watts, voltage is not required.
Can I enter the load in amps?
Yes. Select Amps and enter the battery voltage. The calculator converts the current to watts using volts × amps.
Does the calculator account for inverter losses?
It can approximate them through the system-efficiency field. Enter an efficiency appropriate to the equipment being used.
Is actual runtime always the same as the calculated result?
No. Actual runtime can vary with battery condition, temperature, chemistry, discharge rate, changing loads and equipment behavior.
Using this calculator effectively
The Battery Runtime Calculator estimates operating time from battery capacity and load power while allowing user-controlled adjustments for usable capacity and system efficiency. Capacity can be entered in Ah, mAh or Wh, and load can be entered in watts or amps.
Common uses
- Estimating how long a battery may run a constant electrical load.
- Converting Ah or mAh capacity into watt-hours using battery voltage.
- Comparing runtime scenarios using different usable-capacity and system-efficiency assumptions.
Tips for a better estimate
- Use the battery manufacturer's rated voltage and capacity rather than assuming values from a similar battery.
- Choose usable-capacity and efficiency percentages appropriate to the battery and equipment instead of treating the defaults as specifications.
- Remember that actual runtime can change with temperature, battery condition, chemistry, discharge rate and varying load.
Related calculations
Amps to Watts Calculator · Watts to Amps Calculator · Electricity Cost Calculator