# ASK THE EXPERTS: Peak Sun-Hours

Beginner

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 http://rredc.nrel.gov/solar/pubs/redbook.

Larry Owens • Shoreline Solar Project