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If you put your solar powered math calculator in your backpack it will turn off from the lack of sunlight needed to power the device. As you slowly open your backpack and begin to let sunlight in, the calculator will eventually turn on when the amount of sunlight is enough to power the calculator. Similarly, peak sun hours refer to the hours of they day where the sunlight is strong enough to power a solar panel. This is different from total sunlight hours, which is simply the amount of hours in a day when there is any sunlight.

Although you can easily predict sunrise and sunset hours each day to the minute, looking at latitude can help with your solar planning. The closer you get to the equator the closer your latitude gets to zero. Sunlight hours on the equator are consistent throughout the entire year. Places further from the equator can have large variance in daily sunlight. For example higher latitudes can have very long summer days with lots of sunlight and very dark winters. The latitude of Lowell is 44.8.

Throughout the day the sun obviously moves throughout the Lowell sky. The suns position in the sky also changes throughout the year with the seasons. A fixed solar panel does not accommodate for these changes. However, a 1-axis panel rotates and follows the sun’s path during the day. A 2-axis panel both follows the sun’s daily path as well as the seasonal differences

The sun is a great ball of gas that rises and sets every day that the earth rotates while in orbit around the sun. Barring any major disasters this is a very predictable occurance every day. Latitude helps predict this even more, narrowing it down to the minute for sunrise and sunset. But some things aren’t as predictable that will greatly influence the efficiency of solar panels. Weather and cloud coverage for example can greatly diminish peak sun hours on any given day. Thick storm clouds will block a high percentage of the sun's rays, resulting in lower output of your solar panels. Weather needs to be factored into deciding when to use your system, or how much output one expects to get.

Since we know the latitude of Lowell we can take the average amount of total sunlight hours and estimate that with a fixed solar panel there would be an average of 4.7 peak sun hours per day. 5 hours per day with a 1-axis tracking mount that tracks the sun from sunrise to sunset, and 5.7 hours with a 2-axis tracking mount that tracks the sun everywhere in the sky.