Sun-path charts for different latitudes are available, and each chart includes solar data to show the percentage of sunlight available for each hour of each month. The Solar Pathfinder Assistant software is used with digital photos of Pathfinder readings to generate detailed shading analysis reports ($139; Windows only—Apple computers require running a Windows emulator such as Parallels Desktop for Mac). The Solar Pathfinder Web site hosts several helpful videos on setup, performing a shade analysis, and using the software.
Wiley Electronics Acme Solar Site Evaluation Tool (ASSET) ($599; includes camera, positioning hardware, software, and case) • www.we-llc.com
The ASSET uses software and a digital camera mounted to an adjustable base that’s fitted with a compass. Users level and orient the camera on its specialized mount and take several photos (at set sequential azimuth angles). Photos are downloaded onto a computer, where the accompanying software “stitches” them together in a panoramic view from the perspective of the proposed array location. A sun-path diagram is superimposed over the photo, showing obstructions that may pose a shading threat. Once you have entered your particular “project preferences” (including state and city, camera angle, array configuration, etc.), the software calculates “shading factor data,” reported as the percentage of light shaded for each month and an overall “shading factor” for the location. Just like the Pathfinder, the ASSET can easily be set up in several locations so you can compare your options and find the best location for your array.
Solmetric SunEye ($1,495; includes software, case, and AC battery charger) • www.solmetric.com
The SunEye is a handheld solar site analysis tool that integrates a fish-eye lens, digital camera, touch-screen interface, and software. This device captures whole-horizon images for a site and generates shading analysis reports on the touch screen for instantaneous feedback. When taking a measurement, you first select your state and nearest city, orient and level the device, and then snap your picture. The touch screen can report the percentage of annual sunlight available and summer versus winter sunlight, along with monthly averages. Reports can also be transferred to a PC via the included USB cable and software package (Apple computers require a Windows emulator). A GPS add-on ($295) can obtain latitude and longitude coordinates (±3 meters) for more accurate sun-path calculations.
All three tools include bubble levels and compasses, as they must be leveled and aligned to true south (or north in the Southern Hemisphere) before each measurement. Measurements must be obtained exactly where the array may be placed. For instance, if you are considering a rooftop-mounted array, measurements must be taken from the roof. If it’s a pole-mounted array you’re after, you’ll need to take your measurements from a ladder or scaffold set up at the array’s proposed height. Take readings at each corner of the proposed array location, and at the top and bottom to assess the solar window over the entire area. You may find that the lower portion of the array would be shaded due to objects on the horizon, or that architectural details (overhangs, dormers) or overhanging foliage creates shading problems on the upper portion of the array.
You’ve spotted potential shading in the prime solar window on your property. What now, get out the chain saw? You may have options to either locate the array away from the shaded area or move (or remove) the obstruction. If shading will hit the lower part of your array, you may be able to move the array further up the roof or, with a ground-mounted system, elevate it. You also may want to consider a pole mount. If the eastern side of the array is threatened by shade, perhaps you can locate the array more westward. If trees or bushes are shading culprits, consider trimming (or transplanting) them.
In some cases, reaching for your chain saw can cause other problems. Well-placed trees, like those on the west side of homes, can provide much-needed shade during the summer months—removing them can increase the cooling load on the home and increase demands on your air conditioner or fans. This might negate the increase in energy from the PV array that you gained by cutting down the shading offender. Also, remember that trees and bushes grow, so consider future growth (and shading effects) in your array siting and yard maintenance plans.
The software programs for the Solar Pathfinder, ASSET and SunEye can all simulate the effect that removing or adding an object has on your system—for example, how a solar window could be improved by trimming a tree or how much sunlight would be lost if that proposed power shed is constructed too close to your ground-mounted PV array.