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How much can solar power can I fit on my roof and how much will it produce?  These are two of the most frequent questions that we receive about solar power.

Here are some simple guidelines for starters:

  1. Solar panel’s DC power rating equals approximately 15 watts / square foot.
  2. Most roof areas allow approximately 70% of the space to be effectively used.
  3. In good conditions (no shading and southern orientation) solar power systems in the St. Louis region produce approximately 4 kWh (kilowatt-hours) per day for each kW installed. (This accounts for more production in sunnier months and less production in winter / rainy months over the course of a year.)

So, let’s run through an example:

Available roof space = 15 X 20 feet = 300 ft2

Usable roof space = 300 ft2 X 70% = 210 ft2

Solar on the roof = 210 ft2 X 15 watts/ ft2 = 3,150 watts (3.15 kW – round down to 3kW)

Production = 3kW X 4 hours = 12 kWh / day = 4,380 kWh / year (estimate)