Why solar array sizing calculations use peak sun hours rather than actual daylight hours, what a peak sun hour actually represents, and why the value varies so much by region.
Why Daylight Hours Alone Would Be the Wrong Input
It might seem intuitive that a location with 12 hours of daylight would produce roughly proportional solar output to a location with 10 hours of daylight — but solar array sizing does not use raw daylight hours as an input at all, because daylight hours alone say nothing about the actual intensity of sunlight received throughout the day, which is what actually determines energy production. Peak sun hours is the metric specifically designed to capture this intensity information in a form that plugs directly into a straightforward sizing calculation.
What a Peak Sun Hour Actually Represents
A peak sun hour is defined as one hour during which solar irradiance averages 1,000 watts per square meter — the standard reference intensity used for rating solar panel output. Real sunlight intensity varies continuously throughout the day, rising from zero at sunrise, peaking around solar noon, and falling back to zero at sunset, with intensity also affected by cloud cover, atmospheric conditions, and season. The peak sun hours figure for a given location and time period condenses this entire variable irradiance curve into an equivalent number of hours at the standard 1,000 W/m² reference intensity that would produce the same total daily energy.
Why This Conversion Makes the Sizing Formula So Simple
Because solar panels are rated (in watts) specifically at the 1,000 W/m² standard test condition, expressing a location's solar resource in these same equivalent peak sun hour units allows a direct, simple multiplication: a panel's rated wattage times the location's peak sun hours per day gives that panel's expected daily energy output, without needing to separately model the actual varying irradiance curve throughout each day. This is precisely why peak sun hours, not daylight hours, is the input that appears directly in the standard array sizing formula.
Why Peak Sun Hours Varies So Significantly by Region
Peak sun hours depends on a location's latitude, typical cloud cover, atmospheric clarity, and elevation — regions with predominantly clear skies and high solar intensity (the desert Southwest United States, for example) commonly show peak sun hour values in the range of 5.5 to 6.5 or higher, while regions with more frequent cloud cover (the Pacific Northwest or parts of the Northeast) commonly show meaningfully lower values, often in the 3 to 3.5 range. This is a substantial difference — nearly double between the highest and lowest common US regional values — which is exactly why using an accurate, location-specific peak sun hours figure matters so much more for solar sizing accuracy than using a generic national average.
Why Two Locations With Identical Daylight Hours Can Need Very Different System Sizes
Two locations at similar latitude can have very similar total daylight hours across a year, yet meaningfully different peak sun hours if one location has significantly more frequent cloud cover or atmospheric haze than the other — this is a direct, concrete illustration of why daylight hours alone would be a poor and misleading input for solar sizing, while peak sun hours correctly captures the actual usable solar energy resource difference between the two locations.
Where to Find an Accurate Peak Sun Hours Value
The National Renewable Energy Laboratory (NREL) PVWatts calculator and related solar resource maps provide peak sun hours data with reasonable geographic precision, commonly down to specific ZIP codes or coordinates, reflecting long-term historical solar resource data for that specific location. Using this location-specific data rather than a generic national average — this site's Solar PV Array Sizing Calculator defaults to 4.5 hours as a reasonable national average starting point, but explicitly invites overriding it with an actual location-specific value — is one of the highest-leverage accuracy improvements available for a preliminary solar sizing calculation.
Why This Matters Directly for System Cost
Because required system size is inversely proportional to peak sun hours in the standard sizing formula, using an inaccurate peak sun hours value produces a directly proportional sizing error — understating peak sun hours for a genuinely sunny location oversizes (and over-costs) the resulting system recommendation, while overstating peak sun hours for a genuinely cloudier location undersizes a system that will not actually deliver its intended usage offset. Getting this single input right is worth real attention before finalizing any solar sizing estimate.