Does My Roof Gets Enough Sun to Make Solar Worthwhile?

You are worried!  Your roof has a little shade in the fall.  It gets even worse in the winter.  So what do you do?  HERE ARE SOME SOLAR FACTS:  The average house in the United States consumes electricity at the rate of 1 kW per hour (kWh). There are about 730 hours in each month and for simplicity let’s use an average price of electricity at $.10 per kWh.  (The actual average is $.13.) Using $.10 an average monthly bill would be around $73.00 for 730 kWh of electricity.


A rule of thumb is that each kilowatt requires a minimum of 100 square feet. Since the average residential system is 5 kilowatt (kW) that means that you need at least 500 square feet of roof space. Another way of saying this is that the generating capacity of a typical solar panel is 10 watts/sq. ft. So for every kW you generate, you need about 100 sq. ft. of solar panels.


If the sun shines directly on you for 24 hours a day, you could put up 100 sq. ft. of panels and have enough energy to power the average home.  But the sun does not shine 24 hours each day, so you will have to expand the size of your solar array depending on the location of your home.  Here we are talking about the number of “noontime equivalents” which is the kind of peak sun that beats down on you at noon.  This is called your levels of Solar Insolation.  Now let’s do some quick estimating:

  • If you live in the northern United States you can count on an average of 3 to 4 hours per day of direct sunshine.
  • The central United States showers down an average of 4 to 5 hours of direct sunshine.
  • The southern United States receives on an average more than 4.5 to 6 hours of direct sunshine.


Remember that an important consideration for all roof-mounted solar is the issue of shade.  This is a bigger problem than you may realize because if one panel is shaded it brings down the production of the entire string of panels to the level of solar energy that is generated by the one shaded panel.  However, if the shading occurs only in the early morning or late afternoon you will still be okay because the shading will have no impact on your peak noontime sun.  You will be even better off if the shading occurs only in the winter.  And if you still want to move ahead with solar you can also try out one of the new devices that isolate shading to the one panel that is hindered by the shade.  This device is called a MPPT Optimizer and it protects the rest of the string from degradation of their solar energy generation.


The advantage given to a solar array through the use of this device is powerful because it individually tunes the performance of the panel to match the performance of the string inverter. Power optimizers are also useful when there are differences in the equipment being used. Plus, you can buy “smart” panels that have a power optimizer integrated into them.  What will they think of next!

See You Next Time!  Dr. Stripling


Solar Insolation map of USA