#Peak Sun Hours: From the Solar Constant to Your Roof

Peak sun hours are the unit that turns astronomy into a sizing decision. One peak sun hour equals one hour of irradiance at 1,000 W/m2 - so "5 peak sun hours" doesn't mean five hours of daylight, it means the day's total energy equals five hours at full intensity. Confuse the two and every system estimate that follows is wrong.

The chain starts above the atmosphere. The solar constant is 1,361 W/m2, measured by NASA's SORCE TIM instrument with under 0.1% variation across an entire 11-year solar cycle - the sun is a remarkably steady machine. Earth intercepts roughly 173,000 terawatts continuously; about 30% reflects back to space off clouds, ice, and atmosphere, and 70% is absorbed (NASA's energy budget).

Why doesn't that trivially power everything? Location and timing. Most of the energy lands on oceans - 71% of the surface - where capture is impractical, and peak production at midday doesn't line up with evening demand. Latitude adds its own spread: at 50 degrees north, June days run 16+ hours while December drops to 8, while the equator stays within 30 minutes of 12 hours all year round.

Here's the number I find genuinely startling: covering just 0.5% of land area with 20% efficient panels would meet global electricity demand. The resource was never the constraint. Converting peak sun hours into delivered kilowatt-hours, at your latitude, on your roof pitch, through your winter - that's the actual engineering work.

The articles under this tag do that conversion step by step: how irradiance is measured, how it varies by geography and season, and how to size a system from the hours your site really gets rather than the hours a brochure assumes.