Solar is what turns a power station from a one-shot battery into something that can keep going for days. How fast it actually charges, though, depends on more than the wattage printed on the panel.
The quick formula
Charge time (hours of good sun) = battery Wh ÷ (panel watts × 0.75)
Why 0.75? Panels almost never deliver their rated power. Heat, the angle of the sun, haze, cable losses and the charge controller usually bring real output down to 70–80% of the label.
| Battery | 100 W panel | 200 W | 400 W | 600 W |
|---|---|---|---|---|
| 300 Wh | 4 h | 2 h | 1 h | 0.7 h |
| 500 Wh | 6.7 h | 3.3 h | 1.7 h | 1.1 h |
| 1,000 Wh | 13.3 h | 6.7 h | 3.3 h | 2.2 h |
| 2,000 Wh | 26.7 h | 13.3 h | 6.7 h | 4.4 h |
| 3,600 Wh | 48 h | 24 h | 12 h | 8 h |
Those are hours of strong sun, not hours of daylight. The solar charge time calculator converts them into days for where you live.
Peak sun hours
Even a clear day doesn’t give you 12 hours of full-strength sun, because morning and evening light is weak. Solar installers use “peak sun hours” instead: the equivalent number of hours at full intensity.
- Desert Southwest: about 6–7
- California, Texas, Florida: about 5–5.5
- Southeast and Midwest: about 4–4.5
- Northeast and Great Lakes: about 3.5–4
- Pacific Northwest: about 3–3.5, and much less in winter
So a 1,000 Wh power station on a 200 W panel (6.7 sun-hours) needs about a day and a half of sun in Ohio, and about one day in Arizona.
Your power station has limits too
Every unit has a maximum solar input in watts and a voltage window, something like 11–60 V. Two things catch people out:
- Adding more panels than the input accepts doesn’t speed up charging at midday. The extra only helps when it’s cloudy.
- Going over the voltage limit can damage the unit. If you wire panels in series, add up each panel’s open-circuit voltage (Voc) and stay below the maximum, with some margin, because cold weather pushes voltage up.
Getting more out of your panels
- Point them straight at the sun and tilt them up, roughly to your latitude in degrees. Re-aim every two or three hours if you can.
- Avoid shade completely. A shadow over just a few cells can cut output dramatically.
- Let air flow behind them. Hot panels produce less.
- Keep cables short and thick. Long, thin extensions waste power.
- If your unit allows it, charge from the wall or car and solar at the same time.
Is it worth it?
For backup power, even a small solar setup makes a big difference. A 200–400 W kit can put back 600–1,600 Wh a day in most of the U.S., which in summer is enough to keep a fridge, the internet and lights going indefinitely. To size panels for your daily use, see how many solar panels you need.