Most of us have been in summer mode since Memorial Day, but the first official day of summer arrives on June 21 - the Summer Solstice. That’s also the day when the sun is up the longest, so on average, you’d expect it to be the best day of the year to generate solar power. It’s right up there, but not necessarily the best day because clouds play a role, too.
The graphic above shows how much solar power can be generated from the typical rooftop installation (in yellow) at different times of the year, while also showing the average cloud cover (in blue) during those same times for Philadelphia. It’s no surprise that the sun's high sun angle provides the best opportunity for solar power - but that's only if it’s relatively sunny.
The graphic below highlights a few solar power facts: