Solar battery size calculator
Estimate usable and nominal battery capacity, inverter size, runtime, and budget.
Updated 2026-07-17 · Formula version appears with every result.
Preparing the calculator…
This should only take a moment. You can retry if it does not finish.
Data confidence
Early estimate
Evidence used
Uses backed-up load, runtime, surge, efficiency, chemistry, and depth-of-discharge assumptions.
What can vary
Battery controls, temperature, age, surge duration, and installer design require confirmation.
Planning use
Educational estimate only. Confirm the final design and quotation with qualified professionals.
How this estimate works
Calculations are versioned, reviewed, and reproducible. Inputs are validated and planning ranges are used instead of false precision.
Results support early decisions. Confirm utility rates, equipment specifications, site conditions, and professional requirements before committing money. Review the methodology and source policy before treating any estimate as decision-ready.
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Before using the calculator
What this calculator estimates
Estimate usable and nominal battery capacity, inverter power, and backup runtime from essential loads and battery limits.
Who it is for
Homes and small businesses defining essential-load backup before comparing hybrid or off-grid proposals.
Inputs and methodology
Energy demand equals backup watts multiplied by target hours. Nominal battery capacity adjusts this energy for usable depth of discharge and conversion losses. Inverter sizing considers continuous load and the entered surge multiplier.
Required inputs
- Combined essential running load in watts
- Required runtime and surge multiplier
- Battery chemistry and usable depth of discharge
Philippine planning assumptions
- The essential-load list is complete and does not hide large startup loads.
- Battery controls, temperature, age, inverter efficiency, standby use, and reserve settings affect actual runtime.
Worked example
A 1,500-watt essential load for six hours requires 9 kWh before losses and reserve. At 80% usable depth, the nominal battery must be larger than 11.25 kWh even before adding conversion losses and design margin.
How to interpret the result
Use the output to request an exact usable-kWh statement, inverter continuous and surge rating, protected circuits, recharge assumptions, warranty conditions, and expected runtime at the agreed load.
Use the result as a comparison baseline: change one assumption at a time, keep the original inputs, and ask installers to explain why their written estimate differs.
Common mistakes
- Sizing from total household demand instead of essential loads
- Using nominal capacity as if all energy is usable
- Ignoring motor and compressor startup surge
Related questions
Is battery capacity stated in kW or kWh?
kWh describes stored energy; kW describes power delivery. Both matter, together with surge rating and usable depth.
Why can actual backup runtime be shorter?
Higher loads, startup surges, standby consumption, temperature, battery age, reserve settings, conversion losses, and incomplete charging can reduce runtime.
Limitations and disclaimer
- The result is an educational planning estimate, not a quotation, site survey, electrical design, or engineering approval.
- Verify current rates, equipment, roof conditions, utility rules, and written installer assumptions before spending.
Last reviewed 2026-07-20