Solar panels need sun to generate power, that’s no secret. But how do they fare on cloudy days? When we think of places that are good for solar energy, we usually think of sunny places like California, but you’ll be surprised. Solar power is thriving in places like New Jersey and Massachusetts, which aren’t exactly known for sun. And countries like Germany and the UK actually get a decent portion of their total energy consumption from solar power, despite their cloudy and rainy weather. Even though overcast skies aren’t the ideal environment for solar, generating power during cloudy weather can be done. Employing larger solar arrays coupled with a robust energy storage system is one way to capitalize when sunlight is limited.
Yes, solar panels do work on cloudy days, though they will see a decrease in performance. The reduction in power depends on the efficiency and placement of the solar panels, as well as the intensity of the cloud cover. On partially cloudy days, solar panels may still produce up to 80% of their normal capacity. When the sky is completely overcast, performance may drop to below 50%. Contrary to popular belief, however, solar panels do not need direct sunlight to function.
Cloudy Days vs Sunny Days
Solar panels work best in direct sunlight, as their power output is dependent on the amount of photons that strike the surface. That’s why cities like San Diego and El Paso are great solar cities, they get plenty of sunshine all year round. Efficiency plays a role too. Solar panels with higher efficiencies can convert more sunlight into electrical energy, which definitely helps when sunlight is lacking.
That doesn’t mean cloudy days won’t be productive for your solar array. Cloudy or overcast days aren’t completely dark, there’s still plenty of light, but this light is diffused through cloud cover, making it weaker and less productive for your solar array. On a sunny day, diffuse light may make up only 15% of the light striking your panels, but this number is closer to 60% on overcast days. Diffused and indirect light results in reduced solar panel output. The reduction in the power output is dependent on the intensity of the cloud cover and the efficiency and placement of the solar panels.
If the day is partially cloudy with intermittent sunshine, then you will still produce quite a bit of electricity. As the clouds pass over your home, your power production will dip, causing temporary reductions in productivity. According to the EESI, solar panels can still produce 80% of their maximum output on partially cloudy days. On the other hand, a completely overcast day will seriously dampen the output of your solar panels, reducing productivity by over 50%.
But even if you live in a climate that isn’t exactly known for sunshine, that doesn’t mean you can’t have solar power. New Jersey is one of the top ten solar states in the USA, right behind sunny desert states like Nevada and Arizona. And the UK, known for its rainy weather, still produces 4% of its total electricity consumption from solar energy. Germany, being a bit sunnier than the British Isles, gets just over 10% of its power from solar.
The Edge-of-Cloud Effect
On partially cloudy days, clouds constantly come and go. At the edge of these clouds, the sunlight may be magnified by the water droplets in the cloud. This is called the edge-of-cloud effect. This creates short spikes in electrical output, which might appear to help performance but actually creates issues in large-scale PV production. Solar panels respond quickly to changes in light and shade, and the spikes and dips on cloudy days can cause power surges and large fluctuations, especially if they occur when demand is high. This is why charge controllers and frequency regulation are so crucial to the maintenance of effective PV systems.
The NREL collected cloud data for over a year, studying how clouds affected the spikes and dips in PV output. The data has proved very useful to utilities and solar companies, who have used the study to mitigate the edge-of-cloud effect. Using energy storage and larger solar arrays, utility companies have learned to smooth out the large fluctuations stemming from cloud cover and intermittent sunshine.
Solar panels absorb all light, and they don’t differentiate whether this light comes from the sun or some other source. Albedo is light reflected off surfaces like the ground, nearby walls, or even water. The albedo effect is dependent on the reflectivity of those nearby surfaces. Bright and smooth surfaces reflect more light. Grass, dirt, and asphalt have a lower albedo than concrete, marble, or white paint. Natural surfaces like sand and snow also have a high albedo.
Albedo can significantly increase the incident sunlight on your panels, especially on cloudy days. If your solar panels are placed near surfaces with a high albedo, you can take advantage of this effect and mitigate the reduction in the power output from an overcast sky. Bifacial solar panels collect light on both sides and so are great for taking advantage of the albedo effect.
What About Rainy Days?
Cloudy days often mean rain. Naturally, the rain and increased humidity have an effect on the way solar panels absorb light. The effects can be mixed. Some say the rain can help magnify the light and increase efficiency, while other studies have shown that water droplets on the surface of a PV cell actually reduce performance.
Rain does help efficiency in the long term by removing dirt and debris from the surface. Excessive build-up on the surface of your solar panels can have the same effect as shade or cloudy weather by diffusing the light reaching your panels and reducing performance. (This is why it’s so important to use the right solar panel cleaning tools to keep your panels working at optimum efficiency!)
Tips for Cloudy Days
If you live in a climate known for its not-so-sunny weather, there are several precautions you can take to ensure that you can power through those overcast skies.
Energy Storage: An energy storage system is a must, whether you live in California or the UK. Storing the extra power you generate in deep cycle batteries is necessary for those days when the sun isn’t shining. It’ll also help you get through the rainy season and wintertime.
Keeping your Solar Panels Clean: Dust and dirt buildup on your solar panels increases shading and reduces efficiency. Regular maintenance will ensure that your panels perform their best on cloudy days.
Optimal Panel Placement: Giving your solar panels the right angle and directional tilt for your location will ensure that your panels get the highest possible exposure to sunlight on cloudy days. It’s also important to install your panels away from obstacles, like trees or powerlines.
Microinverters: Microinverters help increase the performance of your solar array on days with intermittent sunshine. String inverters are wired so that any shading will affect the performance of the entire array. With microinverters, only the panels that are being shaded will be affected.
Bifacial Solar Panels: Bifacial panels collect energy on both sides. If placed properly, they can produce up to 30% more energy than single-sided panels. They’re great for taking advantage of albedo and reflective surfaces. This means they perform well in diffuse light due to the increased surface area.
Larger Solar Arrays: Living in a region known for rain and clouds means you have to expect a smaller output compared to the rated capacity of your solar array. A larger solar array will make up for the difference in power output by adding more surface area.
High Efficiency Panels: Higher efficiency panels convert more sunlight into electricity. This is also true on cloudy days when the extra performance can help bring in more energy than lower-end panels.
Unless the sky is a complete blackout, your solar panels will still perform under cloudy conditions. As long as there’s light, your panels will produce electricity. Granted, the diffuse sunlight from an overcast sky will definitely reduce the power output of your PV system. You can still generate a decent amount of energy on partially cloudy days when some direct sunlight is still available. There are ways to mitigate the effects of cloudy weather. A larger PV system, energy storage, and high efficiency solar panels will help you maintain efficiency when sunlight is lacking. Even if you don’t live in a sunny climate, powering your home with solar is still possible. Don’t let a gray sky discourage you from adopting clean energy!
Frequently Asked Questions
Solar panels still generate electricity in cloudy weather, though performance is reduced. The reduction in performance is tied to the efficiency and placement of the solar panels as well as the level of cloud cover.
Keeping your solar panels clean is the best way to maintain efficiency all year round. Dirt buildup reduces the amount of sunlight that reaches the solar cell, which reduces performance. Clean solar panels will perform at their highest capacity, which is necessary during those cloudy days.
New Jersey is a major solar state despite being in the Northeastern US, which isn’t known for sunny weather. The UK and Germany are also large solar producers, even though they’re known for cloudy weather.
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