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What Makes a Roof 'Good' for Solar?

Posted by Adam Sewall In Tuesday, May 25 2010 under: Solar Homes, Solar PV Panels, Rooftop Solar

Updated 2016: This post is as timely as ever. More and more Americans are discovering that they can significantly reduce their electricity bills by installing a rooftop solar photovoltaic (PV) system. In fact, with the right mix of solar rebates, tax credits and a number of other variables -- like net metering and a high unit price for electricity from the grid -- installing a PV system can truly be a great investment.

But before we get carried away, let's review some basics. You may live in a state -- like Arizona or California or Massachusetts or Pennsylvania or Colorado -- where helpful solar incentives are available. You may pay a relatively high per-kilowatt hour (kWh) rate for your electricity, which means that, all else equal, a solar PV system would pay for itself quicker than it would where power from the grid is cheap. You may have even come into some money recently, and are looking for a reliable, medium- to long-term investment. But if you a'int got a good roof, you a'int gonna be doing a rooftop solar installation.

What, then, makes a roof good for solar?

(1) Orientation. Here in the northern hemisphere, southern exposure to the sun is ideal. You can, however, orient panels to the southeast or southwest without substantially decreasing performance. You can read more about the orientation (azimuth) and inclination (altitude) of solar panels here. And, if you really want to geek out, you can estimate your own numbers here.

The roof below, to take an example, has two south-facing sweets spots for solar:

A good roof for solar panels

(2) Age. Relatively new roofs are the best. Simply put, PV panels last a long time -- like decades. You don't want to put panels on an old roof, only to remove them five years down the line to install new shingles.

(3) Roof space. Some people are surprised to hear they don't have enough south-facing roof space to accommodate a PV system of any appreciable size. A very rough rule of thumb: these days, you can fit one kilowatt (kW) of panels on about 100 square feet. Currently, the average U.S. system size is about five kilowatts. So, well, you do the math...

(4) Simplicity. Things like skylights, chimneys, ducts and roof contours can add to the complexity, difficulty and cost of a solar installation. The most straight-forward surface, from a solar design and installation perspective, is a flat, unadorned roof.

(5) Shade. Trees and tall buildings are bad. Simply put, solar panels need sun -- specifically between 9 am and 3 pm -- in order to achieve optimal performance. The profileration of micro-inverters over the last few years (2012-2016) has helped address the shade issue alot. Basically each solar panel in a system has its own micro inverter, setting its own voltage output based on the individual panel performance. So you could have parts of a roof that are partially shaded during the day, and other sections that are clear, and the overall system output would still be pretty good.  But if you are totally shaded, then it still wouldn't work, and you have to find sun, or a community solar buying program (or ground mount in the yard).

In other words, if your property looks like this from space (don't laugh)...

A bad roof for solar panels

...a rooftop solar installation is probably not your best option.

So, if you're looking into a rooftop solar PV system, take a few minutes to ponder these points. While you're hanging around the house this  weekend, take periodic note of the amount of sun hitting your roof throughout the day. If you've got a tall tree that casts a big shadow through the middle part of the day, chance are you'll have to decide between the tree and the new solar panels.

The good news is that we find most homes have roofs that are decent for solar, while only a handful of roofs are not good at all. If you've got a brand-new, unshaded roof, however, with clear southern exposure and a flat, simple design, consider yourself among a lucky few who can rightfully brag about how good their roof is for solar. Not that this will make you particularly popular at this weekend's BBQ...

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