Solar Battery Size Calculator
Estimate solar battery-bank capacity from daily energy use, autonomy, reserve, usable depth, system efficiency and battery-module size.
Default project preview
38.3 kWh
Battery modules
8
Purchased nominal
40 kWh
Estimated usable energy
28.8 kWh
Estimated autonomy
2.4 days
Calculator
Build the material order
Your order will appear here
Enter the project details and calculate a rounded material estimate.
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.
Daily energy demand
Enter this project assumption in kWh/day. The current imperial default is 12 kWh/day.
Battery autonomy
Enter this project assumption in days. The current imperial default is 2 days.
Energy reserve
Enter this project assumption in %. The current imperial default is 15 %. Minimum: 0. Maximum: 50.
Usable depth of discharge
Enter this project assumption in %. The current imperial default is 80 %. Minimum: 1. Maximum: 100.
Battery and inverter efficiency
Enter this project assumption in %. The current imperial default is 90 %. Minimum: 1. Maximum: 100.
Nominal capacity per battery
Enter this project assumption in kWh. The current imperial default is 5 kWh.
Peak simultaneous load
Enter this project assumption in kW. The current imperial default is 5 kW.
Inverter capacity reserve
Enter this project assumption in %. The current imperial default is 25 %. Minimum: 0. Maximum: 100.
Price per battery module
Enter this project assumption in a unitless value. The current imperial default is 2600. Minimum: 0.
How to read the outputs
Battery modules
5 kWh nominal each. Default preview: 8.
Purchased nominal
full modules. Default preview: 40 kWh.
Estimated usable energy
depth and efficiency applied. Default preview: 28.8 kWh.
Estimated autonomy
at entered daily demand. Default preview: 2.4 days.
Inverter capacity guide
peak load plus reserve. Default preview: 6.3 kW.
Battery-module cost
modules only. Default preview: $20,800.00.
Methodology
Transparent calculation
BuildMeter converts every input to a consistent internal unit, applies the selected allowance, and rounds products up to whole purchasable units.
Usable energy need = daily demand × autonomy days × reserve. Nominal bank = usable need ÷ depth of discharge ÷ system efficiency. Modules round up.
Reviewed project guidance
How to use this estimate in a real project
Last reviewed: July 29, 2026
Use this calculator to estimate nominal battery storage from daily energy demand, desired autonomy, usable depth of discharge and system losses. It is a planning model, not battery, inverter or protection-system design.
Calculation steps
- 1Define only the loads that must be backed up.
- 2Estimate daily energy, not just peak watts.
- 3Choose autonomy duration.
- 4Divide by usable depth and efficiency, then compare standard battery modules.
Worked numerical example
A 10 kWh daily critical load with one day of autonomy, 80% usable depth and 90% system efficiency requires about 13.9 kWh of nominal battery capacity.
Purchasing and rounding
Battery systems are sold in modules with specific usable energy, power, voltage and compatibility. Round to whole compatible modules after checking inverter and enclosure limits.
Interpret the result
Energy capacity controls runtime; power rating controls which loads can start and operate simultaneously. Both must be checked.
Page-specific assumptions
- Daily load estimate is realistic.
- Usable depth and efficiency match the selected chemistry and system.
- No seasonal solar shortfall is hidden in the autonomy input.
- Battery degradation is considered separately.
Common mistakes
- Sizing from inverter power alone.
- Using nominal energy as fully usable.
- Ignoring motor starting loads.
- Assuming one average day represents winter production.
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.
FAQ
Solar Battery Size Calculator questions
Why is nominal battery capacity larger than usable energy?+
Depth-of-discharge limits and conversion losses reduce the energy available to loads.
Does this include poor solar-weather days?+
Only through the autonomy-days input. A full design should use local solar production and seasonal weather data.
Is inverter size based only on daily energy?+
No. Inverters must also handle simultaneous and startup loads, which can be much higher than average demand.