Rooftop Mounted Solar PV Systems

Rooftops make the perfect home for Solar PV (photovoltaic) systems. Rather than using up valuable farm land that could possibly used for another purpose such as growing food, place the solar panels on a barren rooftop. In most cases, the majority of electrical power produced, is consumed within the same building, or those next door. This distributed production reduces the load on transmission lines, and produces the most power during the naturally occurring peak load interval.

Rooftop solar PV systems also provide shade which can reduce the need for air conditioning within a building and also prolongs the life span of the roof surface.

As a solar PV system will impact the structure to which it is mounted, it's important to follow local building code requirements. In order to obtain a building permit, most municipalities will require a structural engineer to certify the installation configuration.

 

Flat Rooftop Solar PV Systems

Ballasted Solar PV Array

Flat rooftops can use a ballasted type of mounting system or an attached mounting system. Ballasted systems are most common where a rooftop will hold the additional weight presented by the solar system (mounting, panels, ballast, wind, snow). The amount of ballast (often cement patio blocks) added to the mounting system, is determined by the surface area of the solar modules, roof area covered by the mounts, peak wind speed, etc. Attached mounting systems will either penetrate the rooftop and attached to the weight bearing structure, or may be suspended between the outside walls (to which they are attached). For traditional solar panels (mono or poly) the mounting system allows for an optimal fixed angle. A few of these mounting systems also allow the angle to be changed throughout the year, to increase electrical production.

flat rooftop thin film solar pv

Where a large surface is available, or when additional weight is an issue, thin film solar PV modules may be used. Flat mounting systems for thin film modules tend to provide a very shallow angle towards the sun - mostly straight up. Thin film mounting systems are often very light to ensure minimal weight impacts.

 

Angled Rooftop Solar PV Systems

Angled Solar PV Array

When considering a Solar PV installation for an angled rooftop, the orientation towards the sun is important.

Orientation Towards the Sun

With a flat rooftop, you can point the solar panels at the sun by changing the orientation of the mounting systems. With an angled rooftop you are stuck with the direction you have. Some mounting systems will allow you to make a very small adjustment to the angle. However, by adjusting each panel, you impact the LIFT forces of the wind, snow load, etc. Generally speaking, you are better off to leave the solar panels mounted parallel to the surface of the rooftop.

Here's a link to a program called PVWATTS that will calculate your estimated solar power production by city, for different solar mounting configurations:

PVWatts PV Calculator

angled rooftop solar installation

Using PVWatts, and changing the Array Azimuth, allows us to see the impact of installing a solar PV system that does not face true south (which is different than magnetic south). In my PVWatts example, I used a 1kW system, located in London, Ontario, Canada. Here are the production numbers:

180 degrees - 1152 kWh per year - pointed true south
175 = 1151, 185 = 1149
170 = 1148, 190 = 1143
165 = 1147, 195 = 1135
160 = 1145, 200 = 1129
155 = 1140, 205 = 1122
150 = 1132, 210 = 1112 - 30 degrees off true south
145 = 1123, 215 = 1099
140 = 1111, 220 = 1084
135 = 1099, 225 = 1069
130 = 1085, 230 = 1053
120 = 1053, 240 = 1018
110 = 1013, 250 =  977
100 =  968,  260 =  931
 90 =   918,  270 =  880 - pointed directly East or West

angled rooftop solar railings

From the above figures you can see that a solar PV installation that is NOT facing directly south will still produce 98.5% of full power if it's off by 15 degrees. Even when it's 30 degrees off south, it will produce 96.5% of the maximum power. Once your installation is more than 30 degrees away from true south however, the production numbers start to drop rapidly.

Unless you are close to the equator, you do not want to install solar PV panels on both the East and West side of a rooftop. With the configuration above, this system would produce only 78% of it's potential power. A drop in production of 22%, would likely mean your solar PV system would NEVER pay for itself.

 

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