Site-Specific Solar Resources

We have previously looked at regional solar resources, that is how much solar radiation hits different parts of the US throughout the year. While this provides a baseline, the individual user’s solar resources are going to depend on siting.


Solar collectors, much like solar photovoltaic (PV) cells, require direct, full sunlight. Unlike PV cells, water heaters can still warm water in partial shade, but their efficiency is reduced drastically. Trees, clouds, buildings, or anything else that can cast a shadow over the solar collector should be avoided. Roof-mounted systems are popular because they are often high enough to afford an unobstructed view of the “solar window.” The available kWh/m²/day figures given in yesterday’s post are ideals and measured in full sunlight, so any shading will cut into these numbers.

The solar window is the imaginary box in the sky drawn by the path of the sun on the longest and shortest days of the year (March and December 21st, respectively) for the horizontal borders, and the location of the sun at 9 a.m. and 3 p.m., as shown in the figure. The sun provides any one spot  80-90 percent of its radiation within this window.

Solar window schematic view of southern sky (after Ramlow and Nusz 2010, Figure 7.3).

Where we are, the lowest path of the sun is about 23° over the horizon. You can calculate your own local conditions if you know your latitude and longitude: go to NOAA’s Solar Position Calculator and enter your location, then December 21st at 12 noon, and hit “calculate.” This will give you an approximate lower bound of your window (you’ll have to fiddle around with the time since it is only really astronomical noon at the leading edge of the time zone, so for me in Madison, the sun is at its highest location 15 minutes or so after noon). Repeat the process for June 21st for your upper bound.

You can create measuring devices to get an idea of how far above the horizon this is, or you can do it the rough way: face due south, stretch out your arm out straight south and make a fist, shut one eye and set the bottom of the fist on the horizon from your eye’s point of view, the top of your fist is 10° over the horizon, set your next fist on top, and you’ve got about 20° over the horizon. If you have trees, buildings, or other things above this line, where you are standing is not an ideal location. Try going higher up, if possible. The solar collectors may have to be sited on the ground in an open place farther from your building instead of on the roof. The bottom line is that a clear view of the southern sky is necessary to get optimum results in any given location.

The information for this section comes from Ramlow and Nusz (2010, 130-137).

Ramlow, Bob, and Benajmin Nusz. 2010. Solar Water Heating: A Comprehensive Guide to Solar Water and Space Heating Systems. Rev. ed. Gabriola Island, Canada: New Society Publishers.

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