Solar Panel Size Calculator: How Many Panels Do You Need?

A peak sun hour is one hour of sunlight at 1,000 watts per square meter, the standard test intensity for panels. It is not the same as hours of daylight. A location might have 12 hours of daylight but only 4 to 6 peak sun hours, because early-morning and late-afternoon sun is weaker. Peak sun hours fold the whole day of varying intensity into an equivalent number of full-strength hours, which is exactly what you divide your daily energy by to size an array. Values vary by region and season; see the linked peak-sun-hours table.

Panels come in whole units, and a fractional result means one more panel is needed to fully cover the load. If the math says 1.78 panels, two panels are required; a single 400 W panel would leave you short on an average day and well short in poor weather. Rounding up also restores a little of the margin lost to real-world conditions. If the fractional part is tiny, say 2.02 panels, you might accept the small shortfall rather than add a whole panel, but the default and the safe choice is to round up.

A derate of 0.75 is a sensible default that accounts for panel temperature, wiring and controller losses, dust and module aging, and battery round-trip losses in off-grid systems. Grid-tied systems without battery storage sometimes use a slightly higher figure such as 0.8 because they skip battery losses, while harsh, hot or dusty sites may justify 0.7. Lower derate means more panels. When in doubt, keep 0.75; it keeps the array realistic without grossly oversizing it.

The total array watts you need is fixed by your energy use and sun hours. Panel wattage only changes how that total is split into modules: higher-wattage panels mean fewer of them. The same 711 W target is two 400 W panels, three 250 W panels, or four 200 W panels. Choose panel size by roof or rack space, budget and availability, then let the calculator divide the array target by your chosen panel wattage.

For year-round off-grid use, yes. Peak sun hours drop sharply in winter, so an array sized to the annual average will fall short in December. Critical loads should be sized against the lowest-sun month for your location, which can mean noticeably more panels. Seasonal or summer-only setups, like a camper used May through September, can size to the average or the better months. Use a conservative sun-hours figure for anything you cannot afford to lose.