As a brand-new owner of a solar electric system, of course I want to know how it is producing — whether it is on target for producing 90% of the expected electrical use of the house. The good news is that it’s on track for 100%! I say this with only 6 weeks of data to look at. What makes this estimate seem reasonable is the fact that the Autumnal Equinox is within those 6 weeks.
One of the biggest factors which affects the electrical productivity of a solar PV system is the length of day. The longer the time of daylight, the more energy that is produced. In Seattle day length varies from a maximum of 16 hours in the summer to the minimum of 8 hours in the winter. Midway through the year at the start of Spring and Fall, the day and night is about equal, at 12 hours of each.
You can never really know exactly how much electricity a solar PV system will produce in a year until it has produced it. And it may vary year by year because of other factors, like weather and dust and debris accumulation on the panels. However, in an attempt to predict how our new system is doing, I have taken a sample to extrapolate. I chose Sept 7 through Oct 6, a 30-day period with the Autumnal Equinox in the middle of the time period. The graph below shows the length of day and night on the 21st day of each month and the sample range between the red lines.
During this 30-day sample period, the system produced a total of 433 kWh. The best day produced 19.2 kWh, the worst day only 3.9 kWh, the average was 14.4 kWh. Assuming this period is an “average” month for daylight, by multiplying this value by 12.16 (to get the 365 days of the year), it would seem to indicate the annual electric production potential. This would mean annual production of 5,265 kWh. That’s 100%!!!
Looking at one year of utility bills, the annual electrical use was 5,242 kWh. The solar PV system was originally designed for an annual target of producing 4,536 kWh, or 90% of electrical use. But the installers unexpectedly upgraded a few panels to higher efficiency panels. And could it be that September being historically the driest/sunniest month of the year in Seattle it’s not an “average” month for weather?
Call me an opptimist, but it’s fun to think it may turn out that the house electricity is 100% solar! Net Zero Energy, here we come!