Is Solar Power Doing Its Job?

Recent announcements that the state of Vermont is not meeting its goals for use of alternative energy prompted me to look into how effective the solar cells we installed a couple of years ago have been. In September of 2010 we had an 8.3 kilowatt solar array installed that was expected to generate more than 9,000 kilowatt-hours of power a year, enough to cover most of our electrical needs plus the power used by our family next door.  To estimate the amount of power the array actually generates, I compared our metered power in the last full year before the array was installed, 2009, with the power used in 2011.  Not all the difference is due to the solar array but most of it is, so the decrease in measured usage serves as an estimate of the amount of power provided by the sun in 2011.

To cut to the chase, the array is working at least as well as expected and maybe better. In 2009 our electric meter showed we used 8,735 kilowatt hours of electricity. In 2011 the same meter showed we sent back to the power company a net 4,151 kilowatt-hours of electricity. If all that difference is due to the solar array, it generated 12,886 kilowatt-hours of electricity during the year, about 34.5% more than predicted.

There are several reasons the solar array might have performed better than expected. First, the prediction is based on keeping the angle of the array fixed all year long, but we changed to a steep angle during the winter and a shallow angle during the summer to follow the tilt of the earth as the seasons progressed. (The panels being installed by the Woodstock Aqueduct Company on Route 12 will track the sun automatically, increasing power output by 40% more than a fixed array, according to company vice-president Eric Wegner.)

Second, the prediction by the U.S. Department of Energy assumes an average amount of cloud cover for this area, and 2011 may have been a sunnier year than average here, leading to more solar output. And finally, we tried to use less electricity in 2011 than in 2009, through increased diligence in turning off lights and appliances and in using energy saving devices like CFL’s.

Still, the array is clearly doing its job. The amount of the electric bill we saved, however, is not simply proportional to the amount of power saved, both because CVPS changed their pricing method mid 2011, and because CVPS gives credit for the power generated only up to the amount actually used, not beyond. Even so, the electric bills for the two houses were 77% less in 2011 than in 2009.

By using these savings, together with the cost of building the solar system after rebates, we estimated we had received an annual tax-free return on investment of 6.3%, a lot better return than most banks are giving. Although the size of the return depends on the fact that nearly half of the costs of the system were paid for by federal and state rebates, which come out of our tax dollars, that’s a use for our taxes that directly benefits our planet.

By Norwood Long