Peak sun hours are a simple way to translate the messy real-world sunlight into a number you can use to plan solar production. Instead of counting how many hours the sun is up, peak sun hours indicate how much solar energy a location receives in a day, expressed as the equivalent number of hours at full-sun intensity. That distinction matters because clouds, haze, season angle, and shorter winter days reduce usable energy even when daylight still exists. When people size a solar system only by panel wattage, they miss the biggest variable: how much energy the roof can realistically collect over time. Understanding peak sun hours helps you estimate daily and monthly output, compare seasons, and avoid building a system that looks good on paper but underproduces once real-world weather, roof angle, and losses are accounted for.
Peak sun hours, made practical.
- What Peak Sun Hours Really Mean for Output Math
Peak sun hours are an energy shortcut, not a clock. A day with 5 peak sun hours does not mean you get five straight hours of perfect sunlight. It means the total solar energy received that day is equivalent to five hours at a standardized full-sun level. That lets you estimate production using a straightforward relationship: system size in kilowatts multiplied by peak sun hours gives an approximate number of kilowatt-hours per day, before losses. For example, a 6 kW system in a location averaging 5 peak sun hours might produce around 30 kWh on a clear day, then less once you account for temperature, inverter efficiency, wiring, and shading. This is also why two homes with the same panel count can perform differently, even in the same city, if one roof faces south with minimal shade and a morning tree shades the other. Installers often present this calculation early, and a company like AWS Solar of Los Angeles may describe it as the starting point for turning a monthly utility bill into a realistic system size. Once you understand that peak sun hours represent energy, you can see why season and site conditions matter more than daylight length.
- How Peak Sun Hours Change by Season and Roof Conditions
Peak sun hours shift through the year because the sun angle changes, days shorten in winter, and weather patterns vary by season. Even in sunny regions, winter peak sun hours are usually lower than summer values, which affects production during months when heating loads or electric space heating may increase. This is why annual averages can hide important details. A system sized to cover summer usage may fall short in winter, and a system sized for winter may overproduce in summer unless storage or export credits make that workable. Roof design adds another layer. Tilt and orientation influence how much sunlight is captured during different times of day and different seasons. A west-facing roof may produce more in the late afternoon, which can align with peak utility rates, while an east-facing roof may favor morning production. Shade is also seasonal, since tree canopies and sun angle change shadow paths. Even partial shade on one part of an array can reduce output more than people expect, depending on inverter design and string layout. Peak sun hours help you plan for these variations by shifting the conversation away from panel count alone toward usable energy over a full year.
- Loss Factors That Turn Sunlight Into Real Delivered Energy
Peak sun hours estimate the energy arriving at the site, but system sizing must also account for the energy lost on the way to your meter. Panels produce less when hot, so that summer heat can reduce output even on bright days. Dust, pollen, and bird droppings can lower production, especially in dry months or near busy roads. Wiring resistance and inverter conversion also reduce delivered energy, and those losses are present every day. Shading from vent pipes, chimneys, dormers, and nearby trees can cause consistent dents in production that do not show up in simple averages. Snow, when relevant, can temporarily reduce output until it slides off. Another factor is module mismatch and aging, since panels slowly degrade over time. Because of these realities, sizing often uses a performance ratio or derate factor, a single multiplier that represents typical losses. This keeps estimates grounded. Instead of assuming perfect conversion of sunlight into electricity, you plan for a realistic fraction of the energy to make it through the system. Peak sun hours plus realistic loss assumptions produce a sizing estimate that is easier to trust, and it prevents disappointment when the first cloudy stretch or hot week arrives.
Sizing That Matches Real Sunlight
Peak sun hours matter because they convert sunlight into a usable energy value you can use for sizing decisions. They explain why daylight duration does not match solar production and why seasonal swings can change output even with the same panels on the same roof. When you combine peak sun hours with realistic loss factors, you move from hopeful estimates to practical expectations that account for heat loss, shading, inverter conversion efficiency, and roof geometry. This approach also supports better planning for future loads, such as EV charging, heat pumps, and changing utility rates. A system sized with peak sun hours in mind is less likely to underproduce, less likely to surprise you in winter, and more likely to align with your actual energy goals year-round.
