Solar Electric System Sizing
Step 4 - Determine the Sun Hours Available Per Day

Several factors influence how much sun power your modules will be exposed to:

• When you will be using your system – summer, winter, or year-round.
• Typical local weather conditions
• Fixed mountings vs. trackers
• Location and angle of PV array

We have provided the following charts which show ratings that reflect the number of hours of full sunlight available to generate electricity. Your solar array’s power generation capacity is dependent on the angle of the rays as they hit the modules. Peak power occurs when the rays are at right angles or perpendicular to the modules. As the rays deviate from perpendicular, more and more of the energy is reflected rather than absorbed by the modules. Depending on your application, sun tracking mounts can be used to enhance your power output by automatically positioning your array.

The charts reflect the difference in sunlight during spring, summer, autumn and winter. It is more difficult to produce energy during the winter because of shorter days, increased cloudiness and the sun’s lower position in the sky. The charts list the sun hour ratings for several cities in North America for summer, winter and year round average. If you use your system primarily in the summer, use the summer value; if you are using your system year-round, especially for a critical application, use the winter value. If you are using the system most of the year (spring, summer and fall) or the application is not critical, use the average value.

With the chart and the map, you should be able to determine a reasonable estimate of the sun’s availability in your area.

Insolation Map