microFIT guide: estimate the solar potential of your roof | Searching for Green

Today I’ll show you how a quick way to get an idea of how much electricity you can get from your roof.

The most important thing is the orientation of the roof, followed by its size. To get the maximum power, you need southern orientation. It doesn’t need to point exactly towards south, but the closer the better. In extreme cases, panels can be installed on the east and west slopes of the roof too, but their output will be significantly lower. As for the tilt, at Toronto latitude the optimal angle is around 30 degrees. If you have a sloped roof, panels are usually sitting on a rack parallel to the roof, so this will dictate the angle. Again, almost any slope is OK, since the output is not that sensitive to the angle. For a flat roof, the racks are sloped to the optimal angle, but you cannot use the whole area of the roof, since the first row of panels will shadow the second one, and so on. Speaking of shadows, you must make sure no shadow will go over the panels during the whole day. Objects to look for are trees, chimneys,  vents, antenna towers, buildings, etc. I’m pretty lucky: my roof points almost perfectly to the south and there’s nothing close to it to cast a shadow.

To get the potential power of your roof, go to the free online estimate tool provided by Efan, a solar system vendor [no affiliation, it’s just a useful tool].

  1. enter your address, press the “Find Address” button and you will be provided a Google Maps aerial view of your location. You can zoom in to have a closer view.
  2. select the roof area where you consider installing the panels, by clicking in the corners of the desired place. The tool will draw a shape on your roof, and it will automatically calculate the area. You can move the markers if you want to try different locations (just drag them to the desired position) or clear them all and restart afresh. While you are here, you can also check to see if there are big objects on the east, south and west sides of your roof. I did all the above, and the usable area of my roof is 30 sq meters. When you are satisfied with your selection, press Continue twice.
  3. you will presented a screen to select the roof pitch and the shading level. In my case, 15 degrees and no shading. Press Continue
  4. now select your utility (e.g. Toronto Hydro) and your monthly bill ($100, just for fun). I guess they need these for financial calculations, which we will disregard, anyway :). Press Continue.
  5. finally, you are presented with the results. The only number we want from this screen is the System Power. For my roof, it’s 3 kW, a smallish system but similar to many in the city of Toronto.

I can tell you that the power estimate provided by the above tool is very close to the offers I received form the contractors that visited me, so you can use it as a baseline. The rule of the thumb is that 1kW needs 10 sq meters, unless you have a flat woof or a different orientation. Or, if you like math as much as a root canal, you can order an assessment from OurPower for $25 (no affiliation) or just call a contractor to offer you a free quote. In the end, all that you need is the system power. I strongly advise you to disregard all the financial calculations they are doing. Instead, do them by yourself as I will show you.


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