Having lived with these panels now for over a year, we have a good idea of how they perform. To provide some background, we live in St. Louis MO. Our south wall actually faces about 15 degrees east of south. Our new addition has roughly R-27 insulation in the walls and R-50 in the ceiling, and all windows are double pane. Only two small windows in the addition are not on the south wall. The panels are heating an area of over 900 square feet. There are trees that shade the panels before 9:00 am at the beginning of winter in December. As the sun rises higher in the sky later in the winter, the sun hits the panels a bit earlier in the morning and later in the day.
In mid January on a day with full sun, the panels will begin providing heat about 9:00 am and will run until 2:30 or 3:00 pm. There is a period in the day of about 40 minutes when the panels will stop providing heat due to a shadow cast by a tree in our neighbor's yard. The exact time when the panels come on and shut off depends in part on the outside temperature. Even when the temperature is in the single digits, the panels will provide heat from about 9:20 am until after 2:00 pm. With the outside temperatures in the mid twenties or above, our furnace will not come on at all during the hours in which the panels are operating on days with full sun. Even when the sun is partially obscured by clouds the panels will provide some heat. Our large solar greenhouse does not provide heat until about 10:00 am, but continues to provide heat later in the day. It will heat the old part of the house a bit more than the panels heat the new addition. The old section of our house has only about R-7 insulation in the walls and has more north facing windows. With its large expanse of glazing, nearly 250 square feet, the solar greenhouse easily provides more heat than the small panels. But considering their size the Sunmate solar panels do an amazing job of providing heat.
Overall these panels provide a significant amount of heat reducing the time that our furnace runs. We have put insulated shades on our windows in the addition. These are describe on the window_insulation page. Our main motivation in using solar heat is to reduce our use of non-renewable energy sources. A side benefit is any monetary savings from the reduced use of natural gas. Based on our current cost of natural gas, I estimate that it will take over twenty years before the panels save us enough to cover their cost. Of course, as the price of fuel goes up, the payback period will be shortened. I estimate that we have full or partial sun only about half of the time in our area. Thus, a location with more sun would also see a shorter payback period. My only regret is not installing three panels, two for the family room. This would have provided more heat on partially sunny days and days with temperatures in the teens or lower.
