Size and condition of roof are critical factors in any rooftop photovoltaic (PV) solar system.
Size. Solar panels are generally produced in rectangular sections consisting of solar wafers that absorb the sun’s light and generate direct current. Although every manufacturer differs, these sections are usually about 3 feet by 5 feet or 15 square feet. Therefore, ignoring setback issues discussed below, a rooftop that is 6000 square feet (100’ x 60’) could support some 400 solar panels.
How much electricity would 400 solar panels produce? Today most panels produce between 300-350 watts; panels manufactured 2-3 years ago may produce only 245 watts or so. Accordingly, 400 new solar panels should produce at least 120,000 watts or 120 kilowatts of direct current, roughly 140 kW of alternating current. This is the amount of power that a small grocery might require for lighting and refrigeration.
Unfortunately, we cannot use every square inch of roof space for solar panels. Panels will often need to be set back from the edge of the roof for several reasons: The risk of wind shear, for example, or the need to permit access to HVAC equipment. As a rule of thumb we at Solomon Energy assume that 70% of a clear, unobstructed rooftop area can be used for panels. Of course, if parts of the roof are in shade from, say, elevator shafts or water tanks the calculation will need to be adjusted.
Condition. If your rooftop is in need of repair today, or if you are projecting repairs over the next 10-15 years, it is best to make those improvements today before installing panels. Removing panels is an expensive and time-consuming task. Even if you are contemplating a commercial installation using ballasts to weigh down panels in lieu of installing racks and making roof penetrations, removal can be costly.
Note that roof repairs are NOT eligible for tax credits or depreciation allowance and so cannot be folded into your overall project costs. In some states incentives may be available for roof repairs prior to a solar installation. But these costs must be weighed carefully in your analysis of the long term benefits of a solar system.