Will MY roof work for a PV solar system?

It depends on how much sun you have at your potential site! Of course each house is different, but here are some guidelines which will let you know if a photovoltaic system will work well on your house or business.

First of all, how much sun do you get? You need good sun in order to generate substantial electricity from the sun. It sounds obvious, but I am regularly looking at roofs and potential PV locations which are completely in the shade. Any Photovoltaic system must be in the direct sunlight from 9am until 3pm (at least) for the whole year.

Many folks think that because we are not out in Arizona or California that solar energy is not worthwhile. That is completely untrue. Here in Cumberland county Tennessee we get on average 4.64 hours of strong sunlight per day (averaged over the whole year). In the summer it can be closer to 7 hours, and in the winter it drops to 3 hours or so. But on average, that is very good sun and will allow a photovoltaic system to produce some impressive electricity. For comparison, Germany, the current world leader in photovoltaic systems, receives only 4.18 hours of good sun a day. The strength of our sun can not be ignored on a hot day (especially if one is inside a car in the Walmart parking lot).

Remember in the Winter the path of the sun drops down in the horizon, so there are normally many more shadows in the winter months. The best way to determine if you will be in the shadows is to have your roof analyzed with a “solar pathfinder.” This device is used by solar installers to instantly estimate the amount of shade that a given spot will receive all year round. Contact a solar installer for a shade analysis if you are unsure of how much sun you get at your potential solar location.

The potential Photovoltaic location should be facing as close to due south as possible. The best window for Solar panels is the 45 degree window from SSW to SSE (with due south being the best). To determine which direction your potential site faces simply use a compass.

At our latitude in Cumberland County the best angle for a PV system to be mounted is at a 34 degree slope (around a 12/7 pitch). This angle will allow the panels to produce (on average) the most electricity over a 12 month period. Less sloped systems will produce more in the summer (when the sun is directly overhead and the days are longer), so most roof pitches here in Crossville work fine. Normally we install solar panels so that they match the slope of the roof for aesthetics and because it makes the installation quicker, stronger and less expensive.

One must also consider how any potential roof will support the additional photovoltaic system. The systems weigh about as much as one layer of shingles, so on a new roof, the additional weight is acceptable, but on an older roof with multiple layers of shingles, one should consider re-roofing with shingles during or before the Photovoltaic system is installed. Also remember, the PV systems will last for 35 years, so if they are installed on a roof with only 5 years of useful life remaining there will be added expense when those shingles are replaced because the PV system will need to be removed and re-installed during the re-roofing.

Photovoltaic systems can easily be installed on asphalt shingles, metal roofs, flat top commercial buildings and ceramic tile roofs. Installation procedures are improving and can accommodate almost any type of roof that is currently being used.

Photovoltaic systems can be installed on ground mounts (though they must be protected from traffic), or they also can be installed on specially constructed pole mounts. Most installations these days are roof mounts as the roofs are out of the normal “traffic pattern”, and typically have very good sun exposure.

Shade is the primary consideration when trying to determine if a Photovoltaic system is worthwhile at a given location. Sites with good sun exposure are excellent candidates for Photovoltaic systems. Why not use the energy from the sun that is being freely given every day? The answer of course, is that it is time that we start taking advantage of all that “free” energy.

Randy with a Solar Pathfinder

Randy using a Solar Pathfinder to analyze shade at a potential PV site.

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