Installing PV panels with the right orientation towards the sun is both a simple and complicated process. Simple, because the sun spends most of its time in the southern part of the sky. Complicated, because its trajectory is always changing, and not every rooftop or other array location faces south. Solar professionals refer to the sun's path across the sky on any given day as the solar altitude angle.
Because the sun tracks high in the sky in summer, and low in the sky in winter, trees and other nearby objects may shade the array at some point. So you'll have to take the entire year into consideration when choosing a site. However, your first priority will be the peak season, usually from May to October, when your modules will experience the most daily sun hours and the least amount of cloud cover. Optimum orientation also means setting the upward tilt of your modules to the best angle possible for attracting direct sunlight. This is not always possible, especially for residential PV arrays, which are mounted flat (or parallelt to) the rooftop slope.
At any rate, solar designers calculate the best placement based on measurable sun path data and potential shade obstructions. Following a site survey, he or she (or you) will crunch the numbers and weigh potential shading at multiple locations, tilts and orientations. As illustrated above,the solar window assesses the tract of the sun from Dec. 21st (lowest trajectory) to June 21st (highest). Since the cast of shadows is longer in the wintertime, it's important to assess whether or not your array location will be affected. Later in this section, you'll learn how to do this yourself. For a more comprehensive introduction to sunpath tracking, check out this YouTube video.
One key piece of information designers must determine is the number of daily peak sun hours you can expect for your location. This info allows you to estimate kilowatt production for the proposed solar array. The National Renewables Energy Laboratory (NREL) publishes free data sets online about solar radiation in U.S. cities.
Data sets like this one allow solar designers to calculate the power output of a solar array based on the available sun hours and five different tilt options. In this example, PV panels facing due south in Sacramento get their maximum power generation (5.9 hours a day) by tilting the array at the same angle as the latitude of the city, or 38.5 degrees.To download a chart for your city, use this page.
You can also take advantage of free calculators and interactive forms that help you project annual electricity output for various orientations and array tilt angles. Most solar designes use tools like the Solar Pathfinder and Solmetric Sun-Eye to measure potential shading over a proposed array site for the entire year. We'll look at how these devices are used after reviewing the general concepts of sunpath tracking and array placement.
Mastering the Basics
Here are some rules of thumb to keep in mind when investigating how best to orient an array:
Rule #1: Most residential fixed panels (aka flat plate collectors) generate the most power when tilted at an angle equal to the latitude of your city.
For example, if you live in Los Angeles, which is 34 degrees North (of the equator), you'd want to tilt your array about 34 degrees up from the ground -- assuming you're on flat terrain. Obviously, home roofs are already tilted, so you'd measure your most southerly facing slope to see how close it is to 34 degrees .
One way to measure the angle of the roof slope (aka incline or pitch) is the rise and run method. It requires a level, square, ruler and/or tape measure. You can either stand on the roof, or use the safer alternative of climbing a ladder in front of the side of the house with the roof profile (as shown above). With your tools, creat a right triangle, with the 12-inch measurement parallel to the ground, and and other ruler held straight up to the top of the roof. Here's a video on how to do this. Most rise and run measurements use 12 inches as the run, since what you're looking for is a ratio, rather than actual lengths or heights. Once you've got the rise measured, divide it by 12, then use the arc tan key on a scientific calculator to determine the angle.
If the roof angle is between 30 and 40 degrees, then life is good and you can move on to other calculations. But if the slope's way off the 34-degree mark, you or your designer may have to consider a tilt adjustment. Since mounting rails are generally (or at least should be) bolted several inches off the roof to provide cooling air flow beneath the panels, you can elevate one side of the racking higher than the other . that should compensate for the unfavorable roof pitch. At any rate, the adjustment is known as the array tilt angle.
Rule #2: Position your panels so they face as close to true south as possible.
Whereas a tilt angle involves an altitude (or vertical) measurement, there's also the compass direction to consider. As stated earlier, true south is considered the ideal orientation for fixed solar panels. That's because the sun spends its peak hours shining down on us from that direction, or azimuth bearing as it's sometimes called.
Often in array or roof calculations, south is assumed as the reference direction. Then the bearing given as the number of degrees east or west of south. So for example, 15 degrees east of true south is the same as saying 165 degrees.
These two diagrams indicate that arrays that don't face true south (180 degrees) but are within 45 degrees on either side should work fine. In the lower image, you can see for the location at Edwards AFB, you can still capture 90 percent of available radiation even at a 65-degree variance. The second diagram also takes into account the array tilt or roof pitch (for flush panel installations). Whenever taking a bearing, make sure to compensate for magnetic declination. Click here to generate a diagram for another location.
While not all rooftops have sides that face exactly south, an array orientation within 45 degrees (+ or -) of true south will still generate plenty of power. If, on the other hand, you have sloping rooftops that face east or west (which means they're 90 degrees away from true south), you may want to consider either using two arrays, one on each side, that will combine to produce the energy you need.
The compass direction that solar panels face is known as the array azimuth angle.
Rule #3: To increase electrical output from your solar panels, change the tilt angle every season if possible.
Needless to say, the nature of a "fixed panel" means that the array tilt and azimuth angles remain the same year-round. But many array mounting systems on the market nowadays include a manual "tilt adjustment". So you can increase an array's electrical output by 25 percent or more by making a trip up to the roof three or four times a year with a wrench.
By changing the tilt seasonally, an array's power output can be increased by nearly a third.
First photo: resource-solar.com. Second photo: Goldbeck Solar GmbH
Here's a four-setting adjustment schedule for changing an array tilt angle in mid-latitude locations:
Rule #4: Avoid placing an array where significant shading will block out sunlight.
Shading from a nearby tree or building can be a real showstopper for PV panels. Vents and ductwork on the roof, along with your chimney will all create shadows at some point during the day in one season or another. While a little shading around the edges in winter won't substantially impact the array's production, an entire row of shaded cells in just one module can cause a tremendous power drop throughout the circuit.
To understand how this works, think of your solar panels (or string of modules) as if they were all trains on a subway line. When one train breaks down, all the other trains slow to a crawl. So unless there are safeguards in place to prevent it, overall power production will plummet throughout the array as a result one shaded panel.
Several different tools and devices are available to predict shadows over the course of a year. These include two specially designed gadgets known as the Solar Pathfinder and the Solmetric Sun-Eye. If you don't want to spend the money to purchase them, you can compute the shade manually.
A much longer discussion of how to measure and mitigate against shading is included on Page 3. For links to popular measuring tools, interactive forms, sortware apps and other tools, see our Calculators page.
Rule #5: Consider topography, climate, building codes and energy usage patterns before placing your array.
While the vast majority of solar panels face as closely to true south as possible, it's occasionally better to point an array to the southeast or southwest. For example:
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