Solar panels are a great way to reduce your energy costs and help the environment. But do they work on cloudy days?
The short answer is yes, solar panels do work on cloudy days. This is possible because solar panels can use the diffused sky radiation, but, before delving more into this, you may be asking yourself what exactly is the diffused sky radiation and how can you take the best advantage of it. Let’s find out.3

Direct Normal Irradiance and Diffused Sky Radiation

There are two types of solar radiation that can be used by solar panels: direct normal irradiance (DNI) and diffused sky radiation (DSI).

DNI is the amount of sunlight that reaches the ground directly from the sun. It is the most intense type of solar radiation and is the most efficient for generating electricity.

DSI is the amount of sunlight that reaches the ground after being scattered by the atmosphere. In other words, it refers to sunlight that has been scattered by clouds. In the end, this reduces the amount of direct sunlight that reaches the panels. DSI is not as intense as DNI, but it can still be a significant source of energy for solar panels. Having the ability to harvest energy from DSI is important because, during cloudy days, diffused sky radiation can account for up to 70% of the total solar radiation reaching the ground, which is a lot of energy that solar panels can use to harvest solar power. Solar panels can also make use of reflected solar energy coming from DSI or DNI.

The amount of DNI and DSI that reaches a particular location depends on several factors, including the time of day, the time of year, and the weather conditions. Keep in mind that in general, DNI is highest during the middle of the day in the summer while DSI is highest when there are clouds in the sky.

Figure 1. Solar Radiation Components scattered by the surface and atmosphere. Source: West Virginia University

The Average Percentage of Diffused Sky Radiation from Total Solar Radiation

The average percentage of diffused sky radiation from total solar radiation varies depending on the location. In general, it is higher in cloudy climates and lower in sunny climates.

For example, in the United States, the average percentage of diffused sky radiation from total solar radiation is 40%. In Seattle, Washington, the average percentage is 60%. In Phoenix, Arizona, the average percentage is 20%. Other regions in the world, like the United Kingdom, have higher DSI values that range on average up to 60% and British people still use solar panels in many cases.

How a Solar Photovoltaic System Can Take the Best Advantage of Diffused Sky Radiation

There are a number of ways to improve the efficiency of solar photovoltaic systems on cloudy days. One way is to use a tracking system that follows the sun across the sky. This ensures that the panels are always facing the sun, even when it is behind clouds.

Another way to improve efficiency is to use bifacial panels. Bifacial panels generate electricity from both the front and back of the panel. This means that they can capture diffused sky radiation that hits the back of the panel.
Finally, you can also increase the number of solar panels on your system to cover for unexpected weather conditions. This will help to offset the lower efficiency of solar panels on cloudy days.

Figure 2. Single-Axis Solar Tracking Systems. Source: Sinovoltaics

Conclusion

We have learned that solar panels can work on cloudy days, but not as efficiently as they do on sunny days. However, there are some ways to improve the efficiency of solar photovoltaic systems on cloudy days. By using a tracking system, bifacial panels, or more solar panels, you can mitigate the impacts of shading. So, if you live in a cloudy climate, don’t let that stop you from installing solar panels. You can still generate plenty of electricity, even on cloudy days.