How the recharge time is calculated
sun hours = (capacity × charge %) ÷ min(panel watts × real-world %, max solar input)
Then divide by your peak sun hours per day to get days. For example, refilling a 1,024 Wh station from 10% to 100% needs 922 Wh. With 400 W of panels at 75% (300 W), that's about 3.1 hours of peak sun, or less than a day at 4.5 peak sun hours.
Peak sun hours: rough guide
| Location / season | Peak sun hours per day |
|---|---|
| US Southwest, summer | 6–7 |
| Most of the US, summer | 5–6 |
| Most of the US, year-round average | 4–5 |
| Northern US, Canada, UK, winter | 1–3 |
For your exact location, the free NREL PVWatts tool (US) or PVGIS (Europe and worldwide) give monthly figures.
Getting the most from portable panels
- Face panels at the sun, and re-angle them every couple of hours if you can.
- Avoid partial shade: shading one corner can cut a panel's output by far more than you'd expect.
- Keep them cool and clean. Hot panels produce less.
- Check your station's solar voltage and current limits, not just watts, before wiring panels in series or parallel.
Combine this with the runtime calculator: if a day of sun puts back more than your appliances use, you can keep going indefinitely.