I’ve read that the Seattle area averages only 3.7 peak sun-hours per day. Maybe that’s true in December, but April through October, I’d say it must be more like 10 to 12 hours a day, meaning that the average must be higher than 3.7 hours per day throughout the year. How are peak sun-hours determined?

Jeff Huffman • Brier, Washington

Excellent question! “Peak sun-hours” are not the same as “hours of sunlight.” Sunrise to sunset represents hours of sunlight. But peak sun-hours describe how much solar energy is available during a day.

The daily amount of solar radiation striking any location on earth varies from sunrise to sunset due to clouds, the sun’s position in the sky, and what’s mixed into the atmosphere. Maximum solar radiation occurs at solar noon—the time when the sun is highest in the sky, compared to the rest of the day. Sunlight in the morning and evening does not deliver as much energy to the earth’s surface as it does at midday because at low angles more atmosphere filters the sunlight. Besides day-to-day differences, there are also seasonal effects. In midsummer, due to the sun’s higher position in the sky, an hour of sunshine packs more energy than the same hour of sunshine in the winter.

A peak sun-hour is roughly the amount of solar energy striking a 1-square-meter area perpendicular to the sun’s location over a 1-hour period straddling solar noon in the summertime. So we can compare apples to apples, the amount of power is standardized at 1,000 watts (1 kilowatt) hitting that 1-square meter surface. By adding up the various amounts of solar irradiation over the course of a day, and counting them as units equivalent to 1 solar-noon midsummer hour (1,000 watts per square meter for 1 hour), we get a useful comparison number—the peak sun-hour.

An analogy might help complete the picture. Imagine that you have to pour sunshine into buckets that are 1 meter square, and each holds 1,000 watt-hours of solar energy. The fastest rate of filling that bucket will occur at solar noon in the summer, when the sunlight is really streaming down. At that time, you could fill a 1,000-watt-hour bucket in 1 hour (1 KWH per hour). At any other time of the day, however, it will take longer than 1 hour to get an equivalent “bucket” of 1 peak sun-hour.

On average, summertime Seattle conditions will net you 4.8 peak sun-hour-equivalents from sunup to sundown. Wintertime sees an average of about 2.5 sun-hours per day. Over the course of a year, the daily average works out to about 3.76 peak sun-hours. For month-by-month solar irradiation information for a variety of cities in the United States, visit

Larry Owens • Shoreline Solar Project

Comments (13)

Rio Zak's picture

Hi lads, first, thanks Larry this was REALLY helpful!
I am currently working on a 3 years worth of hourly global solar radiation data in regards to residential PV. My question is that, by what above means, I am supposed to add up all the radiation readings from sunshine down to sunset on daily basis and say I get 3,210 W/m^2/day, it will mean 3.21 of solar peak hours?

Thanks in advance,

Michael Welch's picture

Yes, pretty much. You need to add up the radiation for all days, every day for a year, then divide by 365. That gives you the average daily radiation, which can then be divided to translate to average daily peak sun hours.

But there are some vagaries. For example, how are you measuring this? If you are basing it on the output of a rooftop PV system, your power output will not equal the solar radiation because of inefficiencies in the system from everything from equipment efficiency to dirt or film on the PV modules. Also, when figuring out the array's square footage, you must subtract all the space between cells and modules, as well as total module frame area. Finally, the array would have to track the sun in order to calculate an accurate figure.

Rio Zak's picture

Hi Michael, my apologies for the delay!! Thanks so much for your response, I appreciate it!

No, I was lucky enough to get data from the pyranometer of my town's meteorological station.

Quick question though, is there a conversion factor to translate between the average annual daily peak sun hours and the annual solar irradiation?

Michael Welch's picture

Assuming the pyranometer readings are in watts per square meter, then they are practically the same as sun hours.

Rio Zak's picture

Thanks Big time Michael!! I really appreciate it!

brecht's picture

Hy Paul,

I sent you a mail,


brecht's picture

Hy Woody,

I just found your question when I was searching for other info. Altough it is 7 months old, maybe you still need some info:

I work as renewable energy manager at Comin Khmere in Phnom Penh. We use PVsyst and/or Homer to do sizing of our systems. Both use NASA data for their calculations (which is freely available in tables, but hard to use for singel calculations). If you want you can take up contact with me and I could help you in your design.

Woody Petrea's picture

Hi Brecht, I am ALWAYS happy for assistance. I will make an attempt to contact you through your company website. However, I may be reached at: yahoo (dot) paul (at) gmail (dot) com.

Thanks for the offer of assistance!

Woody Petrea's picture

Hi, I am seeking resources to help verify the average number of sun hours in Cambodia. The city is Sihanoukville. Thanks, for any assistance you may provide. From what I have been able to locate thus far, this number is 4.5?

Justine Sanchez's picture

Hi Woody,

I wanted to add a few free resources for you to check out. PVWatts does include data for some international sites (see right hand side of this webpage):

And while I do not know where their data comes from the Solar Electricity Handbook website does include some data for Cambodia:

Good luck!
Justine Sanchez
Home Power Magazine

Woody Petrea's picture

Hi Justine,

Thanks, as well, for your input.

I truly appreciate the assistance you folks have provided.

Michael Welch's picture

Hi Woody. Sorry that I do not have that information, but you may be able to find it through this resource:

Woody Petrea's picture

Thank you! I will check that out.

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