← Moon Dog · Off-Grid Solar Guides
How many solar panels do I need?
The right number of panels isn't a guess about roof space. It comes from three numbers: how much energy you use in a day, how many hours of usable sun you get, and how much you lose to wiring, heat, and charging inefficiency. Get those, and the array size falls out of one formula.
Step 1 — add up your daily load (watt-hours)
For each device, multiply its power draw (watts) by how many hours per day it runs. Sum them to get your daily load in watt-hours (Wh).
| Device | Watts | Hrs/day | Wh/day |
|---|---|---|---|
| 12V fridge | 45 | 8 | 360 |
| LED lights | 15 | 4 | 60 |
| Laptop + phone | 60 | 3 | 180 |
| Water pump / fan | 30 | 2 | 60 |
| Total | 660 |
Step 2 — find your peak sun hours
"Peak sun hours" is not daylight hours — it's the number of hours per day the sun delivers a full 1,000 W/m². A panel only makes its rated wattage at that intensity, so this is the figure that matters. It varies by location and season: roughly 5–6 in the summer Southwest, 3–4 in the cloudy/winter North. Size to your worst month if you live off-grid year-round.
Why not just use daylight hours? A panel at sunrise or under haze produces a fraction of its rating. Peak sun hours bundle a whole day's varying intensity into an equivalent number of full-strength hours, which is exactly what the math needs.
Step 3 — divide, then add a loss margin
Solar charging is never 100% efficient — expect 25–30% loss to wiring, panel heat, dust, and charge-controller conversion. Divide your load by sun hours, then by a system efficiency factor (use ~0.75):
Array watts = Daily Wh ÷ Peak sun hours ÷ 0.75
For our 660 Wh/day van at 4 peak sun hours (a conservative shoulder-season figure):
660 ÷ 4 ÷ 0.75 ≈ 220 W
So two 110 W panels — or a single 200–220 W panel — covers this van in average conditions. Bump to ~300 W if you want a cushion for cloudy stretches or plan to add devices.
Panels refill the bank each day; the battery carries you through the night and bad weather. Sizing one without the other leaves you short. See the companion guide on battery sizing below.
Watt sizes the whole system for you
Enter your devices and location and Watt returns array wattage, battery bank size, and charge-controller and wire-gauge recommendations — with the peak-sun-hour and loss assumptions shown, not hidden. Built for van, RV, and off-grid builds. Free to download.
Sources
- NREL PVWatts methodology (peak-sun-hour / insolation and system-loss modeling)
- Standard photovoltaic system-design practice (array-to-load and derating factors)
Peak sun hours vary by site and season — size to your worst expected month for year-round off-grid use.