How to Design a Solar-Ready Rooftop

Posted by Erdem Gorgun at

there are two main components to the net- zero house energy equation. There’s conserva-tion: The house needs to be designed and built to require minimal inputs of energy for heating, cooling, lighting, and plug loads. And there’s on-site power generation: No house uses zero power, so to reach net zero, the house has to gen-erate some of its own juice using photovoltaics.In practice, net-zero houses generate surplus energy during the summer, and they consume off-site power during winter and at night. A good balance requires thoughtful consideration of house siting, roof slope, roof orientation, and shading—the key factors that determine a home’s suitability for on-site power generation.Roof OrientationSolar panels produce the most power when pointed at the sun, but since the sun moves, the t y pica l solution is to split the difference. A n idea l solar rooftop in the northern hemisphere will face south—solar south, not magnetic south. The magnetic nor th pole isn’t at the Ear th ’s geo-graphic north pole, so solar south is found using a correction factor to the compass reading.Roof SlopeThe cheapest and simplest way to mount solar panels is on a rack in plane with the roof, where the panels’ angle to the sun is the same as the roof ’s angle. The panels will be perpendicular to the sun’s rays—their highest-output condition—when the roof is. The highest solar output for the year will happen on the days when the sun is shining directly down on the panels.Work With The SiteIn practice, solar orientation doesn’t have to be perfect, says Connecticut builder Nick Lehto, who specializes in net-zero construction. Given the constraints of home building, Lehto can’t always orient his roofs due south, or frame roofs to a solar-optimized pitch. Roof angles and house orientation are import-ant, but “the lot is what the lot is,” he says. “Sometimes you have to accept that solar doesn’t make sense on a project.”With the declining cost of solar panels, builders may try to make up for roof slope or orientation imperfections by adding more panels. “It makes the project more expensive because you’re trying to make up for lower efficiency by throwing more money at it, but it’s doable,” says Charlie Morgan of Eastern CT Solar in Connecticut.The Shading ProblemMorgan notes that shade on the roof can be an issue. “You don’t want any shading on the array, because that will reduce your production,” he says. But advancing technology has reduced the shading penalty. Morgan explains that with the new generation of microinverters, each solar panel acts independently as opposed to the string inverters of the past, where whole strings of panels were wired together. So if one panel is in shade, “it’s only impacting that one solar panel,” he says. “That’s made a big difference in being able to mitigate for some shady situations.”Build With Solar in MindMorgan advises builders to be mindful of the roof ’s structural capacity. He notes that solar installers provide a structural review during the per-mitting process, which includes an official letter by an engineer saying that the roof can accommodate the additional loading of the solar. He also recommends eliminating obstructions on the south side of the roof and limiting skylights where you may install panels. Foresight with regard to wiring connections also can be beneficial in the long run when adding solar, Morgan adds. “There is always going to be some ancillary equipment. ... If you put your electrical panel in a tiny little closet, that might make it tough.”

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