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 — look up your exact location in the irradiance databases below. To size the load side of the system, see the Battery Runtime calculator.
- Peak sun hour definition (1 kWh/m² of irradiation) — NREL solar resource basics
- Regional peak sun hours for Europe — European Commission PVGIS irradiance database
- Regional peak sun hours worldwide — Global Solar Atlas (World Bank / Solargis)
- System losses of 20–30% (75% default efficiency) — NREL PVWatts default loss assumptions plus MPPT controller and battery charge-efficiency datasheets
Assumptions last reviewed on 26 August 2026. If a source disagrees with your battery or equipment datasheet, trust the datasheet.
Frequently asked questions
What is a peak sun hour?
One hour of sunlight at the standard 1,000 W/m² test intensity. A day's varying light is expressed as an equivalent number of these full-intensity hours.
Why is system efficiency well below 100%?
Charge controller conversion, wiring losses, panel temperature and battery charge losses stack up. 70–80% is a common planning figure for MPPT systems.
Can the battery charge while loads are running?
Yes, but any load subtracts from the available charge power, so charging takes longer than this estimate.