On a clear summer day, a 200W camping panel puts about 700 watt-hours into a battery after real-world losses. Under full overcast, the same panel manages about 100 watt-hours, a 7x swing that the "200W" label never mentions. A 12V compressor fridge uses 300 to 500Wh a day. So a 200W panel covers the fridge easily in clear sun and not at all in cloud. Size your camping solar setup for the worst weather you expect, not the best.
The metric: daily watt-hours delivered. This is the energy a panel actually puts into your battery over a full day, after heat, angle, charge-controller and charging losses. It's the number that decides what you can run. The panel's watt rating is only its output in a lab at 1,000 W/m² of light and a 25°C cell temperature, conditions a campsite almost never matches.
Methodology
Model: Daily Wh = rated panel watts × 0.70 net efficiency × peak sun hours × weather factor. Net efficiency (0.70): about 10% heat loss (cell temperature 20–35°C above air on a sunny day, at a typical −0.35%/°C coefficient). Add about 5% for imperfect angle, soiling and cabling, about 5% for an MPPT charge controller, and about 10% for battery charging losses. Peak sun hours: 5 for clear summer days (typical for much of the US in June–July); 3 for clear spring/fall days. Weather factors: partly cloudy = 60% of clear-day output (published ranges are 50–80%); full overcast = 15% (published ranges are 10–25%). What this isn't: a field test. These are modeled figures from published derating ranges, so your exact site will differ. The relative gaps between scenarios are the reliable part.
Rated watts vs. real watt-hours per day
| Panel | Clear summer day | Clear spring/fall day | Partly cloudy summer | Full overcast | |---|---|---|---|---| | 100W | ~350Wh | ~210Wh | ~210Wh | ~50Wh | | 200W | ~700Wh | ~420Wh | ~420Wh | ~105Wh | | 400W | ~1,400Wh | ~840Wh | ~840Wh | ~210Wh |
What the table shows, strongest point first:
- Weather moves output more than panel size does. A 400W array under overcast produces less than a 100W panel on a clear summer day. Buying more panel won't save a trip that runs into three gray days. Battery capacity does that.
- Season is the hidden multiplier. The same clear sky yields about 40% less in spring and fall than in midsummer, because peak sun hours fall from about 5 to about 3. In December, much of the US gets under 3.
- A panel's real daily yield is about 3.5 times its watt rating in good summer conditions, not 5 or 8. The "a 100W panel makes 500Wh a day" shorthand assumes 5 perfect sun hours with no losses.
What that energy actually runs
| Camping load | Typical daily use | Covered by 100W, clear summer? | Covered by 200W, clear summer? | Covered by 200W, overcast? | |---|---|---|---|---| | 2 phones, full charge each | ~30Wh | Yes | Yes | Yes | | LED camp lights, 4 hours | ~40Wh | Yes | Yes | Yes | | Laptop, one full charge | ~60Wh | Yes | Yes | Just barely (with phones/lights: no) | | 12V fan, 8 hours at 15W | ~120Wh | Yes | Yes | No | | 12V compressor fridge, mild weather | ~300Wh | Barely, alone | Yes | No | | 12V compressor fridge, hot weather | ~490Wh | No | Yes | No | | 1,000W coffee maker, 10 minutes (via inverter) | ~190Wh | Only with nothing else | Yes, with room left | No |
The hot-weather fridge figure comes from a Dometic CFX3 45 drawing about 41Ah a day at 12V in hot conditions. Mild-weather use is lower because the compressor cycles less.
Will a 200W panel run a fridge?
Yes, on clear summer days, with about 200Wh to spare for phones and lights. No, on overcast days: 105Wh won't even cover a third of the fridge's daily use. Plan the battery for the cloudy days: roughly 500Wh of usable capacity per day of cloud you want to ride out with the fridge running. For two gray days in a row, that means a power station around 1,000–1,200Wh. That's where the "33% rule" comes in. Solar marketers use it to mean panel watts should be about a third of battery watt-hours (for example, about 400W for a 1,200Wh battery). Fridge makers use a different "33%": compressor fridges run about a third of the time. Both are reasonable starting points. Neither accounts for weather, which the first table shows is the biggest variable. For a full fridge-first sizing walkthrough, see our van life solar sizing breakdown.
Why the box number doesn't happen
Panel ratings assume a cool cell in intense, head-on light. At a campsite, panels heat up and lose roughly 7–14%. They sit at the wrong angle as the sun moves, and pass through a charge controller and a battery charger that each take a cut. Portable folding panels have extra problems: they often lie flat on hot ground and use PWM or USB outputs. We broke those losses down one by one in why your foldable solar panel charges slower than the box says.
Are solar panels worth it for camping?
- Worth it: trips of three or more days, anyone running a 12V fridge, and dispersed camping without hookups. Solar is cheaper and lighter than carrying enough battery for the whole trip.
- Not worth it: one- or two-night trips. A charged battery alone does the job. A ~288Wh unit covers phones, lights and a laptop for a weekend without any panel. Our small solar generator weight-to-runtime comparison shows what each size carries.
- Won't work at all: running space heaters, electric blankets or induction cooktops from solar day after day. Their daily energy needs are several times what a portable array can put back.
The sizing rule
Add up your daily watt-hours from the load table. Choose a panel that covers that total on a clear spring/fall day (the middle column of the first table), not a summer day. Then choose a battery that covers it for as many overcast days as your destination typically gets in a row. For the arithmetic on your own loads, use our solar generator sizing calculator. For panel picks, see our camping solar panel guide.