"I'd think that this time of year, around the equinox, would be pretty close to the annual average."
On the equinox in Somerville, the sun at noon is 42.4 degrees from vertical. At the summer solstice, it.s 20.2 degrees from vertical, and at the winter solstice it's 64.4 degrees from vertical. For a flat panel facing up, the summer solstice is the best time because the sun's rays are only 20.2 degree off-axis. But on a slanted roof, the equinoxes can be the peak season, since a 45-degree angle is pretty much facing the sun directly, like a horizontal panel at noon on the equator.
Basically, different angles have different advantages at different times of the year. Solar panels on a south facing roof with a 45 degree angle will be close to peak efficiency right now and in the fall.
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Date: 2013-04-15 02:16 pm (UTC)On the equinox in Somerville, the sun at noon is 42.4 degrees from vertical. At the summer solstice, it.s 20.2 degrees from vertical, and at the winter solstice it's 64.4 degrees from vertical. For a flat panel facing up, the summer solstice is the best time because the sun's rays are only 20.2 degree off-axis. But on a slanted roof, the equinoxes can be the peak season, since a 45-degree angle is pretty much facing the sun directly, like a horizontal panel at noon on the equator.
Basically, different angles have different advantages at different times of the year. Solar panels on a south facing roof with a 45 degree angle will be close to peak efficiency right now and in the fall.