Battery Backup Runtime Calculator
Estimate battery-bank runtime, usable watt-hours, DC current, batteries required for a target duration and preliminary battery cost.
Default project preview
1.94 hours
Nominal energy
2.4 kWh
Usable energy
972 Wh
Target batteries
5
Estimated DC current
46.3 A
Calculator
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Input and output guide
What each value means
The labels and units below are part of this calculator’s visible methodology. Replace every default with measurements or product data from the actual project.
Battery-bank voltage
Enter this project assumption in V. The current imperial default is 12 V.
Capacity per battery
Enter this project assumption in Ah. The current imperial default is 100 Ah.
Current battery quantity
Enter this project assumption in a unitless value. The current imperial default is 2. Minimum: 1.
Usable depth of discharge
Enter this project assumption in %. The current imperial default is 50 %. Minimum: 1. Maximum: 100.
Inverter efficiency
Enter this project assumption in %. The current imperial default is 90 %. Minimum: 1. Maximum: 100.
Continuous AC load
Enter this project assumption in W. The current imperial default is 500 W.
Energy reserve
Enter this project assumption in %. The current imperial default is 10 %. Minimum: 0. Maximum: 50.
Target runtime
Enter this project assumption in hours. The current imperial default is 4 hours.
Price per battery
Enter this project assumption in a unitless value. The current imperial default is 180. Minimum: 0.
How to read the outputs
Nominal energy
2 batteries before losses. Default preview: 2.4 kWh.
Usable energy
depth, inverter and reserve applied. Default preview: 972 Wh.
Target batteries
for 4 hours. Default preview: 5.
Estimated DC current
at entered bank voltage. Default preview: 46.3 A.
Target nominal capacity
battery-count comparison only. Default preview: 500 Ah.
Target battery cost
batteries only. Default preview: $900.00.
Methodology
Transparent calculation
BuildMeter converts every input to a consistent internal unit, applies the selected allowance, and rounds products up to whole purchasable units.
Nominal watt-hours = volts × amp-hours × battery count. Usable energy applies depth of discharge, inverter efficiency and reserve. Runtime = usable watt-hours ÷ load watts.
Reviewed project guidance
How to use this estimate in a real project
Last reviewed: July 30, 2026
Use this calculator to estimate battery-backup runtime from battery capacity, voltage, inverter efficiency, allowable depth of discharge and connected load. It is a planning estimate because battery voltage and capacity change under load.
Calculation steps
- 1Convert amp-hours × voltage to nominal watt-hours.
- 2Apply allowable depth of discharge.
- 3Apply inverter efficiency.
- 4Divide usable watt-hours by the connected watt load.
Worked numerical example
A 100 Ah, 12 V battery stores 1.2 kWh nominally. At 80% usable depth and 90% inverter efficiency, usable AC energy is about 0.864 kWh. A 300 W load would run about 2.88 hours before additional reserve.
Purchasing and rounding
Battery systems are purchased in compatible modules. Round capacity upward and verify series/parallel configuration, inverter surge rating and battery-management requirements.
Interpret the result
Runtime falls directly as load rises. High startup loads may exceed inverter capability even when average-energy runtime appears adequate.
Page-specific assumptions
- Constant average load.
- Battery capacity stated at a relevant discharge rate.
- Stable inverter efficiency.
- No temperature derating unless included.
Common mistakes
- Using nominal energy as fully usable.
- Ignoring inverter losses.
- Confusing watts and watt-hours.
- Ignoring surge loads and battery-current limits.
Primary and technical references
BuildMeter provides planning estimates, not engineering, architectural, electrical, mechanical, plumbing, permit or code approval. Review the methodology and verify final decisions with product documentation and qualified professionals.
BuildMeter Guides
Understand the measurements and assumptions
Use these visible reference guides before changing project inputs or planning allowances.
FAQ
Battery Backup Runtime Calculator questions
Why is usable energy lower than nominal energy?+
Battery life limits, inverter losses and a reserve margin reduce the energy available to the AC load.
Does connecting batteries in series increase amp-hours?+
Series connection increases voltage while amp-hours remain the same. Parallel connection increases amp-hours. Bank design must use compatible batteries and protection.
Will motors run from the calculated battery bank?+
Continuous energy is only part of the requirement. Motors and compressors can need much higher startup power from the inverter and battery bank.