If you get one number wrong in your solar calculation, let it not be this one. Peak sun hours (PSH) is the single most important location-dependent factor in sizing a solar system, and it’s also the most commonly misunderstood.
Most people confuse peak sun hours with daylight hours. They are not the same thing — and the difference can make your system 30 to 50% too small.
What Peak Sun Hours Actually Means
A “peak sun hour” is one hour of sunlight at an intensity of 1,000 watts per square meter. This is the standard intensity used to rate solar panels — when a manufacturer says a panel produces 600W, they mean 600W under 1,000 W/m² of sunlight.
But sunlight intensity varies throughout the day. Early morning and late afternoon sun is weak — maybe 200–400 W/m². Midday sun in clear conditions approaches or exceeds the full 1,000 W/m². Cloudy periods might drop to 50–150 W/m².
Peak sun hours is a way to compress all that variable sunlight into an equivalent number of “full power” hours. If your location gets 5 peak sun hours, it means the total solar energy received across the entire day is equivalent to 5 hours at maximum intensity.
Peak Sun Hours vs Daylight Hours
A location might have 12 hours of daylight but only 4 to 6 peak sun hours. The remaining hours contribute some energy, but not at full panel-rated output.
This is why using “hours of daylight” as your sun hours value leads to massively oversized expectations. If you assume 10 hours of useful sun and your location only delivers 5 peak sun hours of equivalent energy, your panels will produce half of what you expected.
The opposite mistake happens too. Someone in a desert climate with 7 peak sun hours might conservatively enter “5” because they’ve heard that’s average. They end up buying more panels than they need.
How to Find Your Peak Sun Hours
The best resource is NASA’s POWER database or the Global Solar Atlas. Both provide average daily solar irradiance data for any location on Earth.
Here are some rough averages for reference: the Middle East and North Africa typically see 5.5 to 7.5 PSH, Southern Europe and Mediterranean regions get 4.5 to 6 PSH, Central Europe ranges from 2.5 to 4.5 PSH, Southeast Asia is typically 4 to 5.5 PSH, the southern United States gets 5 to 7 PSH, and northern US and Canada range from 3 to 5 PSH.
These are annual averages. Winter months can drop 30 to 50% below the annual average, while summer months exceed it.
Should You Use Annual or Winter Average?
This depends on your system type and goals.
For grid-tied systems, the annual average works well because overproduction in summer offsets underproduction in winter through net metering.
For off-grid systems, you should use the winter average if you need reliable year-round power. If you size panels for the annual average, you’ll have excess in summer but shortfalls in winter. Using the worst-month PSH ensures your system works even in the least sunny period.
A practical middle ground for off-grid systems in moderate climates is to use a value about 20% below the annual average. This provides reasonable winter coverage without massively oversizing for summer.
How PSH Affects Your Panel Count
The relationship is direct and proportional. A 600W panel in a location with 5 peak sun hours produces 3 kWh per day. The same panel in a location with 3.5 peak sun hours produces only 2.1 kWh per day — 30% less.
To generate the same daily energy with fewer sun hours, you need more panels. There’s no way around this. A system in Germany needs roughly 40 to 60% more panels than the same system in Saudi Arabia to produce the same energy.
This is why entering an accurate PSH value matters so much. A 1-hour error in your peak sun hours estimate translates directly to a 15 to 25% error in your panel count.
Getting It Right in the Calculator
Our Solar System Calculator uses your peak sun hours input for both panel sizing and daily production estimates. Take a few minutes to look up your location’s actual PSH value rather than guessing — it’s the highest-impact improvement you can make to the accuracy of your calculation.
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