How to use it
- Add up your panel watts. Two 200 W panels count as 400 W.
- Look up peak sun hours for your area and season. Free tools such as NREL's PVWatts (United States) and Natural Resources Canada's solar maps give monthly figures.
- Leave yield near 75% for portable panels laid flat or propped at a rough angle. Use less if there is partial shade.
- Enter the energy you need to replace and anything that will be running from the battery while it charges.
The formula
daily harvest (Wh) = panel watts × peak sun hours × yield
net energy = daily harvest − loads while charging
With the starting numbers, 400 W × 4 hours × 0.75 gives 1,200 Wh. Take away 300 Wh of loads and 900 Wh a day goes into the battery, so replacing 1,800 Wh takes about 2 days.
Same panels, different days
| Peak sun hours | Harvest from 400 W at 75% | Net after 300 Wh of loads |
|---|---|---|
| 1 | 300 Wh | 0 Wh |
| 2 | 600 Wh | 300 Wh |
| 4 | 1,200 Wh | 900 Wh |
| 6 | 1,800 Wh | 1,500 Wh |
On a one-sun-hour day these panels only just cover the loads. That is the case to plan for in a winter storm.
Reading the result
- Net energy is positive: the battery gains charge each day. The line below shows how many similar days a refill takes.
- Net energy is zero or negative: the panels cannot keep up. Cut the loads, add panels, or plan another way to recharge.
Limits
- A power station accepts only so many watts of solar. Extra panels above its input limit add nothing, and exceeding the voltage limit can damage it.
- Snow on a panel cuts output to almost nothing until it is brushed off.
- Many lithium batteries will not charge below freezing, so the unit may need to sit somewhere above 32°F (0°C) while the panels are outside.
Common questions
What is a peak sun hour?
One hour of sunlight at 1,000 watts per square metre, the brightness panels are rated at. A cloudy 8-hour day might add up to only 1 or 2 peak sun hours.
Why not use the panel's rated watts directly?
The rating comes from a lab test with the panel aimed squarely at strong light. Real panels sit at imperfect angles, heat up, and lose a little in the cable and charge controller.
Can panels work through a window?
Poorly. Glass, the window's coating and the indoor angle remove much of the output. Put panels outside with a clear view of the sky.
Related
Reviewed October 2026. Results are planning estimates. Check them against your equipment's specifications. Report a problem.