Solar panel wattage ratings represent peak output under perfect laboratory conditions — direct sunlight at the optimal angle with cool temperatures. In real-world RV use, panels typically produce 60-80% of their rated capacity during prime hours, and much less during morning, evening, and cloudy periods. This gap between rated and actual output catches many first-time solar buyers off guard.
The problem with most online solar calculators is they assume consistent peak production for several hours daily. In practice, your 400-watt panel might produce 400 watts for maybe two hours around midday, 200-300 watts for a few hours on either side, and minimal power early and late in the day. Panel angle matters enormously — flat-mounted roof panels lose significant efficiency compared to tilted ground panels, but many RVers choose roof mounting for convenience and security.
Temperature also works against you in ways that surprise newcomers. Solar panels actually lose efficiency as they heat up, which means your panels produce less power on hot summer days when you’re running air conditioning the most. Dark-colored RV roofs can push panel temperatures well above optimal ranges.
A better approach is to calculate your actual daily power consumption, then size your solar array for about 150% of that need. This accounts for efficiency losses, less-than-perfect sun angles, and weather variability. Many experienced boondockers also prioritize battery capacity alongside solar watts — having enough storage to carry you through cloudy periods often matters more than maximum charging speed on perfect days.