The Longest Day (2012~26)

The Longest Day (2012~26)

It won’t surprise you to hear that the longest day of the year is coming up in just a few weeks. In 2012, it looks like June 20th will be a few seconds longer than June 19th and a few seconds longer than June 21st. The Sun goes up and the Sun goes down. When one is in the solar business, this becomes a fairly important part of the day. We think about the Sun. People pay us to think about the Sun. People pay us to harvest as much of it’s light energy as possible and then turn it into something useful, like electricity, and then put that into their homes. At SkyPower, over the years, we’ve actually gotten pretty good at it … in no small part because we think about everyday.

One of those days last week happened on the way home from a new home getting one of our solar arrays. The Sun was setting in the rear view mirror and it “dawned” on us (no pun intended) that the Sun was already set for the folks in the southern part of the city. There was just the last hint of light for the northern city dwellers and then it all of our inverters would be “resting” for the night. We know that the days are not the same length from season to season, but we also know that the days are not the same length depending on where you are. The longest day in Chicago this year will be 15 hours, 13 min and 44 seconds long. That same day here in the Valley will only be 14 hours, 22 minutes and 20 seconds long … 51 minutes and 24 seconds less!

Hmmm? If a summer day in Chicago is longer than in Phoenix and it is more northerly, then it stands to reason that a summer day in northern Cave Creek is longer than that same day in southern Sun Lakes. But how much longer and is it enough to be measured and if it is, then what might that be worth in extra sunlight and consequently, solar electric power? The engineers (geeks) at SkyPower wanted to know … and here’s what we found:

Chicago is at 41.90˚ north latitude. Phoenix is 33.43˚. If one divides the latitude difference by the length of day difference, you’d find that each degree south you travel on the summer solstice shortens your day by 6.07 minutes. The tricky thing about this is that the Valley “recovers” this lost time in the winter when the days here are longer than up north. After all, all days average 12 hours long … but the winter days here use off-peak power while the summer days use expensive on-peak power so it makes a difference in your electric bill.

Cave Creek is 0.61˚ farther north than Sun Lakes and enjoys an extra 3 minutes and 42 seconds of Sun each day in June (a minute and half at dawn and another minute and a half at dusk). At 16.5¢ for each kilowatt hour at on-peak rates, this difference equals 0.51¢/day of extra power if you live in the northern Valley. There are 183 days of extra time in the north … worth a total of $0.93 and 183 days of extra time in the south worth $0.31. Subtract it out and you’ll find that a house in northern Scottsdale gets 62¢ more power from the Sun each year than the same house in Gilbert!

Except for one thing (actually two), dust and monsoon storms. It might be dustier in the south because that is where the haboobs start … but it might be cloudier in the north because that is often where summer monsoon storms begin. It’s all very hard to know for sure. There are so many factors that go into the actually production of power and length of day is just one of them.

The next time you are driving at dawn or dusk here in the Valley (or anywhere), it might be fun the consider that the day is longer or shorter by a measurable amount in places that you can actually see … and when that daylight is collected for electric power, that means money!

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