The first step is to draw the roof section that you would like to add solar panels to. You ideally want to avoid roof sections that face north (as they won't generate much power). 

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Click (or tap) to start the shape. Then click again to add a point. Keep clicking (or tapping) to add points. Click on the first point to close the shape. 

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Top tip: don't draw around the whole roof. Just add one section at a time.