Do solar panels work on cloudy days?

In these winter days, when the hours of sunshine are shorter and clouds cover the sky, one of the most frequent questions from our customers is how can our solar panels be affected on cloudy days with hardly any sun?

In this article we will review how our solar panels work, the elements that influence the performance on cloudy days and we will see how we can improve our photovoltaic self-consumption on days without sun.

Do solar panels work on cloudy days?

The answer would be, yes, our solar panels will still work on cloudy days. It is common to think the opposite, since the initial intuition leads us to associate photovoltaic energy with the sun’s heat, and it is important to know that this is not the case, but that our panels are powered by sunlight and not by its heat.

While clouds can limit direct radiation from the sun, they do not totally prevent light from reaching our solar panels. The energy absorbed by our solar panels comes from both visible light and a range of different wavelengths, many of which can penetrate the cloud cover without a problem. This is known as diffuse radiation, and occurs when the sun’s rays do not strike our solar panels directly, having to pass through clouds or fog.

So as long as there is light in the environment, the panels will continue to generate energy, whether the day is cloudy or sunny.

However, the effect of very cloudy or completely overcast days, we will see in the amount of energy that our photovoltaic modules are able to produce, that is, in their performance.

What elements influence the performance of solar panels on cloudy days?

Photovoltaic panels are composed of solar cells that collect sunlight and convert it into direct current (DC). This current is sent to the inverter to be converted into alternating current (AC) which is the electrical energy that we usually consume in most homes. The efficiency of a solar panel would therefore be the percentage of sunlight that the panel collects and converts into the actual electricity we consume. The greater the amount of energy produced, the higher the efficiency of our photovoltaic modules.

There are different factors that can influence this performance. In the case of cloudy or overcast days, two in particular stand out: the technology of the panels and the type or thickness of the clouds.

In terms of technology, the cells that make up our solar panels are the ones that convert the sunlight that strikes them into electricity (photoelectric effect). Larger cells will be more effective in capturing sunlight than smaller ones. Similarly, there are other technologies that can be more efficient than others, such as bifacial panels, which can collect more solar radiation throughout the day.

Beyond the photovoltaic modules, the performance is also influenced by the other components of the installation, such as the inverter, the wiring and the battery, if available.

As for clouds, not all types affect performance in the same way. In this sense, low, dense clouds with different degrees of opacity (nimbostratus and stratus or haze) are the ones that will limit the performance of our solar panels the most.

How much is the performance of our solar panels affected on cloudy days?

To see how cloudy days affect the performance of our photovoltaic modules, we have used the monitoring of a residential installation in Madrid and an industrial installation in the south to compare the electricity production on a cloudy day and a sunny day, both in winter and summer. We first sought to compare two consecutive days of sun and clouds, and then compared the cloudiest day of the month with the sunniest.

For the residential self-consumption installation, the production on January 19, 2020, with clear skies, was 10.79 kWh. The next day, the skies were overcast and production dropped to 2.08 kWh. This means that the output on the cloudy day is 80.73% lower than on the sunny day.

If we compare the cloudiest day of that month with the sunniest day, the yield drops to 96.34%.

If we place ourselves in the month of July 2020, the 7th was a sunny day, being able to reach a photovoltaic production of 16.36 kWh. On the following cloudy day, the production dropped to 9.54 kWh. This represents a yield loss of 41.69 %. A reduction that amounts to 44.6 % if we compare the cloudiest day with the sunniest day in August 2020.

For the industrial self-consumption industrial self-consumptionThe production on January 19, 2021, a sunny day, amounted to 33.70 kWh. Meanwhile, on the 20th of the same month, the skies were cloudy and production fell to 9.47 kWh. This represents a 71.90% loss in performance. And if we compare the cloudiest day with the sunniest day of that month, the percentage loss rises to 87.79%.

Finally, if we look at July 2020, the sun shone on the 11th to reach a production of 74.22 kWh. The next day, the skies were overcast again and production dropped to 52.68 kWh. This makes a yield loss of 29.02 %, which would increase to 35.12 % if we compare the most cloudy day with the clearest day.

Thus, depending on the type of cloud cover we encounter, the yield of our solar panels can vary between 5 % and 70 % of their capacity on a cloudless day. The loss will be greater on winter days, due to the fewer hours of sunlight available compared to summer.

Can cloudy days help improve the performance of our solar panels?

In the winter months, if we add to a cloudy day that the days are shorter and therefore, there is not much possibility of balancing the loss of performance with the time that our panels are exposed to sunlight, clearly the answer would be no.

However, if we are in summer with temperatures exceeding 25°C during the day, the output of solar panels can be affected by temperature (as we saw in another blog post). Some types of clouds, especially white fluffy clouds, can help to reduce the temperature of the solar panels and thus contribute to improve their performance.

In addition to reducing the temperature, these types of clouds act as a mirror by reflecting the light that bounces off the earth’s surface or, in this case, also on our panels. In this way, they receive direct sunlight and the light reflected by these clouds.

However, we must understand this improvement as relative, because although with white and fluffy clouds we can reach higher peaks of energy production, sunny days are still the days with the highest total production throughout the day.

How to improve the efficiency of solar panels on cloudy days?

Although technologies are advancing rapidly and there are already prototypes of cells that improve the normal performance on cloudy days, the best way we currently have to take advantage of our photovoltaic installation on days when the production of the panels drops considerably, is to get an energy storage system or solar batteries. In this way, you can take advantage of the surplus production on sunny days to use them at night or on days when the performance falls due to external causes to the installation, such as overcast skies.

Another possibility, if you have a self-consumption installation connected to the grid, is to dump the surplus energy, asking your utility company to compensate you for the surplus in your electricity bill.

In case you have an isolated photovoltaic installation, it can be useful, in addition to the batteries, a backup generator, especially when we chain several very cloudy days.

Photovoltaic self-consumption systems are built to withstand the elements, both physically and in their ability to produce energy, under any normal weather conditions, whether rain, wind, hail or clouds.

If you are thinking of switching to photovoltaic self-consumption, be clear that most of the inclement weather will not have a great impact on your clean energy installation, with which in addition to saving you will be contributing to reduce, among other things, CO2 emissions.

And if you need advice or a no obligation quote, contact us without obligation. We will make a preliminary study in which we take into account the average solar radiation in your area so that your installation adapts both to your consumption needs throughout the year and to the production possibilities of the area where the installation will be carried out.