A battery runtime estimate helps you check whether a battery can support a device, backup system, or portable setup for the required duration. The calculation converts the battery’s amp-hour rating into usable watt-hours, then compares that energy with the load’s power demand. It is an estimate, not a guarantee of real-world performance.
How to Calculate Battery Runtime (step by step)
Step One: Calculate the battery’s rated energy
Multiply battery capacity by battery voltage to estimate the battery’s total energy.
Battery energy = battery capacity × battery voltage
Battery energy = 100 Ah × 12 V = 1,200 Wh
Step Two: Apply the usable depth of discharge
Multiply total battery energy by the depth of discharge expressed as a decimal. The default depth of discharge is 80%, which becomes 0.80.
Usable energy = battery energy × (depth of discharge ÷ 100)
Usable energy = 1,200 Wh × (80 ÷ 100) = 960 Wh
Step Three: Divide usable energy by load power
Divide usable watt-hours by the load’s wattage to find runtime in hours.
Runtime = usable energy ÷ load power
Runtime = 960 Wh ÷ 100 W = 9.6 h
Step Four: Convert hours to days
Divide the runtime in hours by 24 to express the result in days.
Runtime in days = runtime in hours ÷ 24
Runtime in days = 9.6 h ÷ 24 = 0.40 d
What your results mean
The default inputs produce an estimated runtime of 9.6 hours, or 0.40 days. This assumes the battery can deliver its rated capacity at the stated voltage and that the load continuously uses 100 watts.
Actual runtime is usually lower. Inverters consume energy, batteries lose performance with age and temperature, and some battery chemistries should not be discharged as deeply as others. High starting currents can also affect devices with motors, compressors, or other surge-heavy loads.
For a more realistic estimate, use a conservative depth-of-discharge value and account for the efficiency of any inverter or converter between the battery and the load. If the load changes over time, calculate each operating period separately or use its average power demand.
Quick comparison
A larger capacity increases runtime directly, while a larger load decreases runtime. For example, doubling battery capacity doubles the ideal runtime, but doubling load power cuts the ideal runtime in half. Battery voltage matters because amp-hours must be converted into watt-hours before runtime can be estimated.