Solar calculator
Estimate how many design-solar days an array needs to move a battery from its starting SOC to the target, with array and charging losses shown separately.
Calculation breakdown
| Energy to store | 2,048 Wh |
|---|---|
| Stored energy per solar day | 2,736 Wh/day |
| Equivalent peak-sun time | 3 h 22 min |
Solar days = energy to store ÷ (array W × peak sun hours × efficiencies) = 2,048 ÷ (800 × 4.5 × 0.8 × 0.95) = 0.75 solar days
Method
The battery's SOC change is converted to watt-hours at nominal voltage. Daily stored energy is array watts multiplied by peak sun hours and both solar-system and battery-charge efficiency.
Clouds, shading and seasonal resource change the calendar time, so the result is expressed in design-solar days and equivalent peak-sun hours.
Energy to store = V × Ah × SOC change Stored Wh/day = array W × peak sun hours × solar efficiency × charge efficiency Days = energy ÷ stored Wh/day
Symbols
Worked examples
Example
Charge from 20% to 100% with 4.5 peak sun hours, 80% solar efficiency and 95% charge efficiency.
Energy = 12.8 × 200 × 80% = 2,048 Wh Stored/day = 800 × 4.5 × 80% × 95% = 2,736 Wh Days = 2,048 ÷ 2,736 = 0.75
Result: 0.75 solar days.
Limits of the model
Assumptions
Peak sun hours and efficiencies represent the design conditions.
Not covered
Does not model charge-stage taper, controller clipping or hour-by-hour weather.
Sources
Next steps
Turn a daily energy target and local peak-sun resource into array watts, then round up to whole panels without hiding system losses.
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Last meaningful update: 2026-08-19. This date changes only when the model, the sources or the guidance change.