As the nights start to close-in and days continue to get shorter you might notice that your solar panels are not performing as well as they did in the height of summer. Don't worry they haven't broken, they're just not getting as many sun hours during the course of a day. With the sun at a much lower angle in the sky, and for less hours, your solar panels are not going to be able to capture as much solar energy to convert into electrical power.
Average amount of full sun hours across the United States
(based on the yearly average)
Depending on where you're sailing in the country you will see a big difference in the average peak sun hours and also notice a big difference between summer and winter. Eg. someone sailing around Seattle, WA gets around 30% less sun hours in the summer compared to someone sailing around Miami, FL
High | Low | Average | |
Portland, ME | 5.23 | 3.56 | 4.51 |
Miami, FL | 6.26 | 5.05 | 5.62 |
Los Angeles, CA | 6.14 | 5.03 | 5.62 |
Seattle, WA | 4.83 | 1.60 | 3.57 |
How does this affect my solar panels?
Solar panels have a rated wattage based on a certain amount of solar irradiance, when you look at the potential daily yield from a panel you look at the number of sun hours. A good average is 4.5.
So a 100W panel with 4.5 sun hours will give you 450Wh (watt hours) in a day. If you then want to convert that into Amp hours you would divide by the charge voltage of your battery bank:
So for a 12V nominal battery bank - 450Wh / 14V = 32Ah
How to calculate daily yield?
Here's a link to our blog post that goes into a lot more detail about how to calculate daily yield from your solar panels.
Summary
Sun Hours are factored into system sizing for residential & commercial installs as they want to make sure they offset your usage in the winter aswell as summer. Sun hours can be useful info for boaters to understand how their panel performace varies over the year but to be honest doesn't come into system sizing.
More often than not on boats the major factor in system sizing is the amount of "roof space" available for solar panels.
More often than not on boats the major factor in system sizing is the amount of "roof space" available for solar panels. You maximize the amount of solar panels you can physically fit and that's what you have to work with. Hopefully it comes close to 100% offset of your power usage aboard or if not it will greatly reduce the number of engine hours just for battery charging.
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