Solar Energy Production - Part 2

Seasonal Changes

My solar panels have been operating for six months, long enough to go from the June summer solstice to the December winter solstice. The panels have now experienced a full range of seasonal solar altitude and azimuth changes from maximum to minimum. As expected, energy production steadily diminished from June thru December as daylight grew shorter and solar altitude decreased.

The panels became operational on June 19th, one day before the summer solstice. Clouds, of course, affect power output, so I found the clearest possible cloud-free days to track seasonal changes. The first nearly cloud-free day after June 19th was June 26th when the panels produced their biggest amount so far, 20.63 kwhrs as shown in the following image. Some thin clouds appeared in late afternoon on June 26th, so I'm sure the absolute summer solstice maximum is slightly greater than 20.63 kwhrs.

The autumnal equinox happened on the morning of September 22nd. This day was, conveniently, completely cloud-free from sunrise to sunset! Energy production on September 22nd was 17.25 kwhrs as shown below.

The winter solstice happened on the morning of December 21st. Unfortunately, December 21st was completely overcast. December 26th, five days later, was the completely clear day closest to the winter solstice. Energy production on December 26th was 11.81 kwhrs as shown in the next image.

When no clouds interfere, energy production curves are nearly symmetric about their peaks except for small diminishing tails on the right side just before sunset. You can see the curves narrow as daylight hours decrease. Energy production is greatest while the Sun is near meridian transit around noon. At the summer solstice there is a broad flat maximum peak value at 0.60 kwhrs. At the autumnal equinox peak production is still at 0.60 kwhrs, but the flat peak has narrowed. At the winter solstice there is no flat peak, and the peak maximum value has dropped to only 0.50 kwhrs.

I've given examples of the best possible days for solar energy production. What about the worst days? The worst day so far happened on November 12th, a dark, completely overcast day with rain. In spite of heavy black clouds, dim light generated 0.56 kwhrs for the day as shown below.

I can imagine even lower energy production on a cold, dim, snowy day near the winter solstice when snow covers the panels from dawn to dusk.

Based on solar panel performance during the past six months I should have installed fourteen panels instead of ten. Fourteen panels would generate close to my usual yearly energy use without exceeding actual energy use. I'll be investigating what's involved in adding four more panels soon.