Seasonality of Solar

Posted on 10/14/2019 at 12:00 by Samuel Jones


Seasonality of Solar Title Graphic

It's that time of year again. 


Leaves are changing colors, there's a brisk chill in the air, and unfortunately for solar, the sun doesn't shine as long.


You might have wondered, how do shorter days affect solar?


Well, as I'm sure you can imagine, they don't produce as much energy. 


Many environmental factors affect the amount of light your panels will see per day, including weather, latitude, panel orientation, and changing seasons.


In general, as you travel north (or south in the southern hemisphere), changing seasons will have a more significant impact on daily solar production. 


At higher latitudes, panel orientation becomes critical since the sun is not always at the same angle in the sky throughout the year.  


It is vitally important to know and understand these factors when designing a solution. 


A solar-powered device that you only test during the summer in Arizona may fail if installed in Seattle during the winter. 


How do you account for the environment when designing a solar solution? 


The National Renewable Energy Lab (NREL), created PVWatts, an online solar output calculator that does exactly that. 


It is designed for grid-tied solar installations but can be applied to off-grid portable solar as well. 


It uses historical irradiance and weather data collected all around the world to determine the average daily amount of light that will reach your solar panel at different times of the year.


For off-grid applications, you need two pieces of information, location, and panel orientation. All other fields can be ignored. For some applications, these are very simple, but others may be more complex. 


The general rule of thumb is to orient the panel based on your latitude. In the continental US, that could be anywhere from 25 degrees in Florida to 50 degrees near the Canadian border. 


Panel orientation becomes more complicated if your panel is fixed and installed in different regions around the globe. 


For these cases, choose a tilt angle somewhere in between the two latitude extremes of the areas where your panel could is deployed. Optimizing for the northernmost extreme can also be a good strategy since, in general, these regions get less sun. 


Here is an example of how the PVWatts calculator can be used for off-grid solar applications. 


Let's say I have a wireless sensor product that monitors agriculture fields, and I would like to power it with solar. The sensors could be deployed anywhere in the continental US. I know that each device requires 250mWh of energy per day to operate.


How much solar do I need, and at what angle should I mount it?


Opening the PVWatts calculator, the first input is the location. 


Since my product could be deployed anywhere in the continental US, I will compare Seattle Washington and San Antonio, Texas, which are close to the extremes of my use case. 


We can ignore all system information except tilt since the only number we care about is average daily irradiation. I will use south-facing 45-degree tilt, which is optimized for deployment in the Northern US.


The results are summarized in the tables below.


Average Daily Irradiance in Hours Seattle Washington


Average Daily Irradiance in Hours San Antonio Texas


As we can see, the level of irradiance changes a lot throughout the year. We can also see that panels in the Southern US will get a lot more light than panels up north, especially in the winter.  


If we design the solutions to work in Seattle, Washington, in December, then we can be confident it will work anywhere in the continental US. 


We will see 1.85 hours of full sun equivalent light per day in Seattle, Washington, in December. That means we need a panel that will produce 300mWh in 1.85 hours. 


250mWh / 1.85h = 135mW Solar Panel 


This is the minimum amount of solar needed to keep your device running and is a good starting point when determining what a solar solution might look like for your application. 


Once you have this number, you can start accounting for other factors, such as converter losses and safety margins.


Creating a solar solution isn't as simple as designing a solution, and it working flawlessly no matter the time of year. 


Solutions need to be designed intentionally factoring in applicable variables and, like the above example, planning for the worst-case scenario, ensuring that your solution will perform no matter what.


If you have an application that could be powered through solar, please contact us


We can help you think through these variables and find the best possible solution to fit your needs.


Categories: Solar Education