How solar charge time is calculated
First work out the energy missing from the battery: nominal capacity in watt-hours times the share still to be filled. Then reduce the solar array's rated power by a system efficiency to get the effective charge power, and divide energy by power. The result is time in peak sun hours; dividing by your location's peak sun hours per day converts it to days.
For example, a 100 Ah battery at 12 V holds 1,200 Wh, so filling it from 50% needs 600 Wh. A 200 W array at 75% system efficiency charges at about 150 W, so the estimate is 600 ÷ 150 = 4 peak sun hours — 0.8 days when a day delivers 5 peak sun hours.
Why real charge time can differ
The v1 model treats a peak sun hour as constant rated power, but real irradiance varies with weather, season, shading, and panel angle. Battery charging also slows near full charge — lead-acid absorption can add hours, and lithium chargers taper in the last few percent. Loads running during the day extend the estimate further. Treat the result as a clear-sky planning figure.
Typical peak sun hours per day
| Location and season | Peak sun hours |
|---|---|
| Northern Europe, summer | 4–5.5 |
| Northern Europe, winter | 0.5–1.5 |
| Central Europe, summer | 5–6 |
| Southern Europe, summer | 6–7.5 |
| Desert climates | 7–8.5 |
Methodology
The calculator UI, the panel comparison table and the displayed formula all call the same calculation function. Peak sun hour figures above are indicative planning ranges; production versions should attach named irradiance sources and review dates. To size the load side of the system, see the Battery Runtime calculator.