A Winter Day in the Life of a Solar Panel
Dave and Helen Damouth www.damouth.com
9 January, 1999
Notes about this chart:
Data taken on January 9, 1999 at San Simeon State Park, California, 35° 35.7' North Latitude. The day was mostly sunny, with occasional high cirrus clouds.
The array consists of three Siemens SP-75 photovoltaic panels, tilted up to approximately face the height of the sun at noon. The manufacturer's rated load current for these panels is 4.4 amps each, or 13.2 amps total - very close to what is actually achieved. The axis of the panels was pointed slightly east of south, so that the panels were pointed directly at the sun at about 11 A.M. (that's the way the RV site happened to point). Note the asymmetry this creates, with a very rapid rise of current in the morning and a slower tail in the afternoon
The current dip between 10 and 11 a.m. is due to high, thin, cirrus clouds drifting across the sun. The abnormal peaks at 9:55 and 11:07 a.m. are due to additional solar power reflected from clouds near the sun but not actually covering it. Smaller amounts of cirrus appeared at other times, causing other deviations from a smooth curve. With the panels flat on the RV roof, maximum current is about 61% of the above values.
Integrating the current curve produces the cumulative amphours curve. I have some reason to suspect that the digital ammeter in my solar charge controller may read about 10% high, so it is possible that all the above numbers should be reduced by 10%.