Interpreting the NREL Insolation Data

So once you have located your city or a city close to you, the data will look like something similar to this:



This table shows the solar insolation values for a fixed tilt array facing true south. There are also tables provided for single or double axis tracking arrays. For most situations, especially in Arizona, it may not be worth considering a tracking array. Financially, the APS/SRP rebate favors a larger fixed array versus a smaller tracking array (see Incentives & Costs page for information about the APS/SRP rebate).

For most grid-tied applications, you'll want to concentrate on the average annual solar radiation column circled above. In the example above, a south facing array tilted at an angle of approximately 20° (35° latitude – 15° = 20°) will receive an annual average of 5.8 Peak Sun Hours per day. If you are installing your array flat on a roof, we have a table of common equivalent roof pitches:

	Roof Pitch	Tilt Angle
	4:12	18°
	5:12	22°
	7:12	30°
	12:12	45°
	21:12	60°

So what happens if your roof isn't facing true south? Well, it turns out that as long as your roof pitch is at a 45° angle or less, you will only lose a maximum of approximately 10% even if your roof faces ESE or WSW. Doug Livingston, of the Solar Living Institute has provided the following chart for Arizona to help with these calculations:

Tilt	Flat	15°	30°	45°	60°	Vertical
Roof Pitch	Flat	15°	30°	45°	60°	Vertical
South	0.89	0.97	1.00	0.97	0.89	0.60
SSE, SSW	0.89	0.97	0.99	0.96	0.88	0.59
SW, SE	0.89	0.94	0.95	0.90	0.83	0.59
ESE, WSW	0.89	0.92	0.91	0.87	0.79	0.57
E, W	0.89	0.87	0.82	0.75	0.66	0.48

For example, if you have a home in Flagstaff that has a 4:12 roof pitch and it faces SouthWest (SW), you will receive 94% of the available insolation. Therefore, if 6 peak sun hours is the maximum average for Flagstaff, the calculation would look like this:

6.0 x .94 = 5.6 Peak Sun Hours


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