Calculating the Amount of Solar Radiation

You've probably noticed that the sun's path across the sky varies depending on the time of year. During the winter, the sun is lower in the sky. During the summer, it's path is more directly overhead.



Here in the northern hemisphere, the sun's east-west path across the sky occurs to our south. In order to maximize the amount of sun we receive throughout the day, we orient our solar arrays to the south. More specifically, the optimum orientation will be true south. This allows the modules to produce power the entire day. To determine the direction of true south in your area, you need to determine your magnetic declination. For Flagstaff, our magnetic declination is 13° east of magnetic south. This would be about 167° on your compass.

The optimum tilt angle of your array will depend on your location, but the best average system output will be achieved when your tilt angle is equal to your latitude. For example, Flagstaff is at 35.13 degrees latitude. This means that the optimum tilt angle for your array would be approximately 35 degrees:



The Willow Bend Center pictured below has an almost ideal roof for a PV installation. Because it is a passive solar structure, the slope of the roof faces almost exactly true south. The roof has a 6:12 pitch, which is approximately 27°. Because the tilt angle is less than the optimal 35° for Flagstaff, it simply means it will produce a little more during the summer and a little less during the winter.



The amount of power available from the sun will vary greatly depending on your location. If you are anywhere in the Southwest, you can count on a great solar resource. The amount power available from the sun is called insolation. Insolation is measured in Watts per square meter (W/m²). The maximum insolation available is generally accepted to be around 1000 W/m². This will vary depending on your location, elevation, etc. Maximum insolation is typically achieved during "Peak Sun" or "Noon Sun" conditions when the sun is directly overhead. Therefore, over the course of a day, the amount of insolation received from the sun will vary. It will typically start out small, gradually increase until it peaks around noon, then slowly decrease as the sun lowers in the sky.

So as the amount of insolation varies, so does the output of your solar system. So in order to determine just how much insolation your site will receive, we need to take a look at the historical data. The National Renewable Energy Laboratory (NREL) has historical test data available for cities throughout the US. This data is measured in terms daily average hours of maximum insolation or "Peak Sun" equivalent. For example, the average daily Peak Sun hours as calculated by NREL in Flagstaff is 6. This means that the amount of insolation received on an average day in Flagstaff is equivalent to 6 hours of "noon sun" or "Peak Sun" conditions. This makes completing our system sizing calculations very easy because we can assume our daily system output is equivalent to our maximum array output for six hours. Therefore, if we had a solar system that had a 1,000 Watt array that was perfectly efficient, it will produce an average of 6,000 Watt-hours (6 hours x 1,000 Watts) per day. Unfortunately, solar system components are not perfectly efficient, but we'll discuss that in a later section.

So in order to determine the amount of insolation you receive in your area, you can go to the following link and click on your state to find data on a city near you.

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

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