Inputs and units
Daily use is in watt-hours, not watts. A 50 W load running for 10 hours uses 500 Wh. Choose bank voltage, days of autonomy and usable depth. For the panel estimate, enter peak sun hours and the selected system efficiency. Peak sun hours represent equivalent full-sun energy, not simply hours between sunrise and sunset.
How the calculation works
Bank capacity (Ah) = daily Wh × autonomy days / (bank V × usable fraction)
Panel rating (W) = daily Wh / (peak sun hours × system efficiency)The bank calculation reserves the part of nominal capacity that you do not intend to discharge. The panel calculation spreads one day’s energy requirement over the chosen solar resource and applies a separate efficiency factor. In this implementation, that efficiency selector changes the panel estimate only. If daily use is measured on the AC load side, account for conversion and other losses in the energy budget before relying on the bank result.
Worked example
For 1,000 Wh per day, two days of autonomy, 12 V and 50% usable depth, the result is 333 Ah. Nominal storage is 4 kWh, of which 2 kWh is allocated as usable. Four peak sun hours at 75% system efficiency gives a panel estimate of about 333 W. That panel estimate covers one ordinary day of energy; it does not add capacity to rapidly recharge a bank depleted over several days.
Assumptions and limits
The method does not simulate weather sequences, charging limits, shading, battery temperature, ageing or seasonal demand. The chemistry labels contain planning fractions rather than guarantees for every battery. Follow the specific battery’s charging-temperature and operating limits; generic notes cannot account for heated or specially managed packs. Panel design and charge-controller sizing require voltage and current checks beyond a watts-per-day estimate.
References and calculation details
Page prepared September 18, 2026. Examples describe this calculator’s implementation; reference links do not imply independent certification.