As solar panels continue to drop in cost, integrating photovoltaics into a house roof continues to be a more and more attractive option. For homeowners who want net-zero or nearly net-zero performance, the question of whether a house is suitable for solar becomes particularly important.
There are two components to the net-zero house energy equation. On the one hand, there’s conservation: The house needs to be designed and built to require minimal inputs of energy for heating, cooling, lighting, and plug loads. On the other hand, there’s on-site power generation: No house uses zero power, so to reach net zero, the house has to generate some of its own juice using photovoltaics.
In practice, net-zero houses generate surplus energy during summer months when days are long and the sun is high in the sky, and they consume power from off site during winter months (and, of course, at night). To reach 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 generation of power.
Solar panels produce the most power when they’re pointed directly at the sun. But since the sun moves across the sky from sunrise to sunset, the typical solution is to split the difference. To maximize production, an ideal solar rooftop in the northern hemisphere, including the United States, will face dead south—solar south, that is, not magnetic south. There’s a difference. The magnetic north pole isn’t at the earth’s geographic north pole, so to find solar south, you apply a correction factor to the compass reading. The factor varies by geographic location; local solar installers are familiar with the required correction to compass readings in their areas.
The cheapest and the simplest way to mount solar panels is on a rack in plane with the roof. In that case, the angle of the solar panels to the sun is the same as the roof’s angle. The panels will be directly perpendicular to the rays of the sun—their highest-output condition—when the roof is. So ideally, the roof will be framed at that pitch. The highest solar output for the year will happen on the days when the sun is shining directly down on the face of the solar panels. The roof angle determines which days that will be.
What’s Good Enough?
In practice, however, solar orientation doesn’t have to be perfect, says Connecticut builder Nick Lehto. Lehto specializes in net-zero construction, building airtight, double-wall wood-frame homes in the New England–farmhouse style. “I try to get to a HERS rating of about forty before PV, and then at that point I implement a PV system to bring the house down to zero,” says Lehto.
But given the real 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 important, he says, but not always under the builder’s full control.
“I try to maximize it the best I can given the lot,” says Lehto, “but it doesn’t always work out. The lot is what the lot is. Sometimes you have to accept that solar doesn’t make sense on a project. But in a wide-open field, we always try to orient the roofline, as well as a lot of the windows, towards the south, and I shoot for roof pitch of 8/12 to 10/12.”
What about the difference between magnetic south and true south? Lehto says, “To be honest, I don’t pay too much attention to that. From what my solar installer tells me, if you’re off a few degrees, the penalty isn’t all that significant.”
Thomas Tutor, a solar design specialist with ReVision Energy in Maine, confirms the point. “One of the common misconceptions is that a solar roof has to face due south. Of course, if you’re building new, and you have a choice, orienting it due south is ideal. But anywhere within 45 degrees east or west of that is very good.” A house facing 45 degrees off of due south still produces 90% of its potential power, says Tutor.
Charlie Morgan, of Eastern CT Solar in North Stonington, Conn., handles Nick Lehto’s solar panel installs. Morgan says, “Most of the houses I do are retrofit projects. I think the same is true of most solar installers. So you have to work with what you’ve got. A lot of times that roof is not oriented toward perfect south. That doesn’t mean it’s not a good roof for solar. Solar south is ideal; once you start moving east and west of south, you start losing some production and some efficiency in the system.”
But non-south-facing roofs can still produce a lot of power. Says Morgan: “If you had an optimum south-facing roof, you’d be at 98% production; if you had an east-facing roof at a 35-degree pitch, you’d be at 80% production. And if you were east at a 21-degree pitch, you’d have about 84% because there’s less shading by the roof. So that flatter roof helps you out a bit.”
Thomas Tutor points out, “With a zero-degree pitch—so, a flat roof—it doesn’t matter which angle it’s facing. That’s sort of obvious, but you get the idea: At a flatter pitch, the home’s orientation angle is less important.” For houses that don’t face due south, a flatter roof can be an advantage, Tutor explains: “For example, if you have a 20-degree pitched roof, and you are facing true south, you are at 96% solar tilt orientation factor. If you went down to a west roof, you are at 80%. But that 80% is better than a 40-degree pitched roof that is also due west. Because that flatter angle can still catch a little bit more of the sun’s rays when it is to the south.”
Given the declining cost of solar panels, builders may find it practical to make up for imperfections in roof slope or orientation by simply adding more panels where space allows. Says Morgan, “We often do that. 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. And with the price of solar panels not the limiting factor that it was, adding a few solar panels doesn’t generally break the bank anymore.”
The Shading Problem
Shade on the roof, on the other hand, can be a significant issue. “The big bugaboo now when you’re trying to design a system and optimize production is less the orientation and more making sure that you’re not encumbering yourself with any shading from trees,” says Morgan. “You want at least six hours of open sky. We kind of use the window between 9 a.m. and 3 p.m. on a south-facing roof. You don’t want any shading on the array, because that will really reduce your production.”
Advancing technology has reduced the shading penalty. Explains Morgan, “Back when we were using string inverters, you would wire up a string of 6 to 12 modules in series, and if you had another dozen panels, that would be another string. But that way, if one panel in a string was impacted by shade or being dirtier than the other panels or something, it would bring down the production of that whole string of panels. But with the new generation of micro-inverters, each solar panel acts independently. If one solar panel is impacted by shade, it’s only impacting that one solar panel and not the rest of them. So that’s made a big difference in being able to mitigate for some shady situations. But regardless, if you’re impacting one solar panel with shade, you are still losing some production. Keeping the shade off the array is important.”
A newer innovation is the “DC optimizer.” Says Tutor, “These are little pieces of electronics that go on the back of each panel and they help regulate the voltage so that they don’t cause one panel having shade to affect the output of panels beside it. DC optimizers are similar to micro-inverters, where you do have individual panel-level electronics, but the DC optimizer isn’t actually inverting the power there on the roof, so it keeps that more sensitive piece of electronics in the basement (the inverter), which we find is more effective and has a lot fewer service calls and issues than with the older micro-inverters that we used to install.”
There are a couple of ways to evaluate a building site’s shading situation, says Morgan. “We have a tool that we can go up on the roof with, called a Solmetric SunEye (solmetric.com). It takes a fisheye view of the sky, and you can calculate the amount of shading potential for different times of the year. But even easier than that now is a software that is available that allows you through satellite imagery to model the irradiance on a given site remotely using Google Earth. So I can take a Google Earth picture, draw my array on it on the computer, and then add in the trees, and the software will calculate the shading and the production for me. That’s called Aurora (aurorasolar.com).”
Building with Solar in Mind
With the cost of solar panels continuing to drop, some home buyers are deferring solar purchases. As a solar installer, Morgan has a few other words of advice for builders who are building with solar in mind, either now or in the future.
“Keep in mind the structural capacity of the roof,” Morgan advises. “One of the things we have to do is provide a structural review as we go through the permitting process—basically, a letter signed and stamped by an engineer saying that the roof can accommodate the additional loading of the solar.”
Wiring connections are another consideration. Morgan says, “It’s really nice to provide a wiring chase down from the attic to wherever the load center or interconnection point is going to be. There is always going to be some ancillary equipment, such as the inverter, that goes along with your stuff on the roof. It’s nice to have room to put that. If you put your electrical panel in a tiny little closet, that might make it tough.”
ReVision Energy likes to be involved in the project from the beginning, says Tutor. This ensures that conduit runs are suitable for the purpose. He explains, “We run into issues where contractors have put in non-electrically-rated conduit, or it’s got multiple bends and there are not the pull points that it needs, or it terminates in the wrong spot, or it’s inside a wall where we can’t find it.” ReVision typically installs 1-inch-diameter EMT conduit, says Tutor. Code requires 3 feet of space on either side of the electrical panel and in front of it, he notes; the same requirement applies to the inverter for the solar system, so that additional space needs to be provided.
It’s also wise to eliminate obstructions on the roof. “Make sure your vent pipes for the plumbing in the house are on the non-south side of the roof,” advises Morgan. “Limit the number of skylights where you’re planning to maybe put in your solar panels. And avoid putting dormers on the south side of the roof.”