How much solar and battery do I need?
Start with measured or estimated daily energy. Battery storage covers the load for the selected autonomy period; the solar array replaces daily consumption during the available peak sun hours. A recharge target adds extra array capacity to recover a discharged battery while still serving the ongoing daily load.
Solar and battery sizing formulas
Required delivered energy = daily load × autonomy days
Requirement with headroom = required delivered energy ÷ (1 − planning headroom)
Nominal battery energy = requirement with headroom ÷ maximum usable fraction
Battery Ah = nominal battery Wh ÷ nominal voltage
Minimum solar watts = daily Wh ÷ (peak sun hours × solar efficiency)
Calculated example
For 1,000Wh/day, one day of autonomy, 80% maximum usable fraction, 0% planning headroom and a 12.8V nominal battery:
How to interpret the result
Nominal battery capacity is the nameplate energy before the maximum usable fraction is applied. Planning headroom adds a separate sizing margin above the delivered-energy requirement, so it is not a second depth-of-discharge control. A 12V-class lead-acid preset uses about 12V for the Ah calculation; a 12V-class LiFePO4 preset uses 12.8V. Chemistry changes the usable-fraction starting point but does not silently change voltage.
The solar result is a planning minimum based on average peak sun hours; it is not a guarantee for every day. When a recharge period is entered, the model adds the battery deficit divided by recharge days to the ongoing daily load exactly once. Seasonal irradiation, shading, temperature, controller losses, panel orientation and weather can all require a larger array.
Use measured loads where possible. The watt-hour calculator can turn appliance watts and operating time into Wh/day. Use the solar panel angle calculator for a latitude-based tilt starting point.
Solar and battery calculator FAQ
How do I calculate battery size for solar?
Multiply daily energy by autonomy days, apply planning headroom, divide by the maximum usable fraction, then divide nominal Wh by battery nominal voltage to find Ah.
How many watts of solar do I need?
Divide daily energy by peak sun hours and solar system efficiency. Add recovery energy if the battery must recharge within a set period.
What are peak sun hours?
Peak sun hours express daily solar irradiation as the equivalent number of hours at 1,000W/m².
How many days of battery autonomy do I need?
It depends on weather risk, backup expectations, generator availability, space and budget. One to three days is a planning range, not a universal requirement.