Solar radiation: How it varies year to year

Solar radiation: How it varies year to year

April 30, 2024 Uncategorized 0
Solar Radiation | Wolf Track Energy

Living in the Northland, we know that some seasons are sunnier than others. The sun sets much earlier and rises much later in the winter compared to the summer. In other words, the amount of solar radiation hitting the Earth varies by season.

But did you know some years are sunnier than others? While solar radiation doesn’t vary too much year-over-year, it can differ by about 10%.  

For folks who installed solar panels at their home or business, that can make some years more productive than others.  

That’s why solar system owners shouldn’t despair when they have a tough month of cloudy weather – eventually it evens out and stays consistent year–over–year. After all, solar is a marathon, not a sprint.  

Gloomy January

Solar owners probably noticed that January 2024 was an especially down month for solar production. In fact, it was the gloomiest month on record, according to the Minnesota State Climatology Office.  

“January 2024 finished as the least-brilliant January on record at St. Paul, with less solar radiation (sunlight) than any other January since 1963,” the state said.

Using the National Solar Radiation Database from the National Renewable Energy Laboratory, we can see just how much variation there is irradiance from month to month and year to year.  

The database records the amount of Global Horizontal Irradiance falling in particular areas. GHI is “total amount of shortwave radiation received from above by a surface horizontal to the ground.” It includes irradiation coming both directly and indirectly from the sun. That makes it “of particular interest to photovoltaic installations,” or solar energy systems.  

The GHI data I retrieved shows the watt per square meter recorded every hour. That helps show how much solar power is hitting Duluth over the course of a month, and it illustrates how much irradiance can vary over time.  

Annual irradiance totals varied by as much as 10% between 2022 and 2018, according to the latest data available. The swing was even wider when I examined the monthly totals.  

Perhaps not surprisingly, the shoulder seasons of September and April saw significant differences between the high and low totals – both saw nearly 50% swings. July was consistently the sunniest month, and December was the gloomiest on average.  

Sunspot cycle 

The amount of solar irradiance hitting the Earth can also be affected by what’s known as the “Solar Cycle.” 

“Sunspots are dark areas that become apparent at the Sun’s photosphere as a result of intense magnetic flux pushing up from further within the solar interior,” according to the National Oceanic and Atmospheric Administration.  

The number of sunspots fluctuates in a predictable 11-year pattern known as the Solar Cycle. The solar maximum occurs when there’s a lot of sunspot activity, and a lull is known as a solar minimum.   

Wrapping up 

We love living in Minnesota in part because we experience all four seasons here. But those seasons can be different every year. This winter, for example, saw much less snow than the year prior, which was good for solar system owners. But the gloomy weather in January dampened solar production.

At Wolf Track Energy, we encourage customers to take the long-term view of their solar system. It’ll take time for it to pay for itself, so one bad month won’t make or break your investment.

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