The Solar-Ready Rooftop

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.

Roof Orientation
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 U.S., 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; solar installers are familiar with the required correction to compass readings in their local areas.

Roof Slope
The cheapest and simplest way to mount solar panels is on a rack in plane with the roof. In that case, the solar panels’ angle 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.

Work With The Site
In practice, however, solar orientation doesn’t have to be perfect, says Connecticut builder Nick Lehto. Lehto specializes in net-zero construction, building air-tight, double-wall wood-frame homes in the New England farmhouse style. “I try to get to a HERS rating of about 40 before PV, and then at that point I implement a PV system to bring the house down to zero,” says Lehto.

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, 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? According to Lehto, “From what my solar installer tells me, if you’re off a few degrees, the penalty isn’t all that significant.”

Charlie Morgan of Eastern CT Solar in Stonington, Conn., handles 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 more flat roof helps you out a bit.”

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 once was, adding a few solar panels doesn’t generally break the bank any more.”

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 AM and 3 PM on a south-facing roof. You really 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 six to 12 modules in series, and if you had another dozen panels, that would be another string. And because they were wired up like that, if one panel in a string was impacted by shade or being dirtier than the other panels, it would bring down the production of that whole string of panels. But with the new generation of microinverters, 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.”

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. It takes a fisheye view of the sky, and you can calculate the amount of shading potential for different times of the year." He also recommends software called Aurora, which works with Google Earth images and will calculate the shading and energy production for the project's solar array.

Build With Solar in Mind
For builders looking to include solar in projects now or in the future, Morgan advises they always keep in mind the structural capacity of the roof. As a solar installer, “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,” he says.

Another area where foresight can be beneficial when prepping for solar is a project's wiring connections. “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, whether it is an inverter that works with your power optimizers or, in the case of microinverters, you have multiple circuits that get combined somewhere down near your main electrical panel. There is some extra equipment, monitoring equipment and so forth, 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.”

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,” says Morgan. “Limit the number of skylights where you’re planning to maybe put in solar panels. And avoid putting dormers on the south side of the roof.”