For some, investing in solar panels in the unsettled climate of the British Isles might seem counterintuitive, but it is still a worthwhile investment. However, weather does impact the performance of solar panels, read on to find out how a solar panel system in the UK will perform come rain or shine.
Solar Panel Performance in Rainy Weather
Contrary to popular belief, solar panels still generate electricity in rainy weather, though it will be at a reduced rate due to cloud cover. Rain can actually be beneficial for the long term performance of the panels as it can rinse off dirt and grime when the roof is pitched. We did an in-depth dive into the impact of rain on solar panels in this blog.
Cloud Cover and Solar Panel Output and Efficiency
Cloud cover will generally have a negative impact on solar panel output as it reduces the amount of sunlight that makes it onto the surface of the solar panel. The drop in output will vary a lot depending on the nature of the cloud - up to a 90% drop with dense cloud cover, say in a thunderstorm, though much less with lighter cloud cover, especially with high altitude cirrus clouds which will have the least impact. Generally, it is possible to tell the impact hour to hour from your solar monitoring system, and as a rule of thumb, although solar panels do pick up a greater spectrum of light than the human eye such as UV light, the brighter and the more direct the sunlight, the better your solar panel output.
Although the amount of cloud cover can vary considerably over a single day, the good news is that the solar panel system output responds almost instantly to any changes in sunlight and is able to handle shifts in light to maximise performance. Additionally, it is said that every cloud has a silver lining and in the case of solar output it is that clouds can sometimes improve output when there are some clouds in the sky but the sun is not obscured. This is because a cloud can reflect light from the sun and cause the amount of irradiance hitting the solar panels to increase; although this is likely to be negligible on your annual PV kWh output.
So Does Greater than Average Cloud Cover Decrease Output When Compared to a Quoted Annual Estimate?
At Spirit we estimate a given amount of energy (kWh) will be generated from a certain kWp system per year. This is based on historical averages for annual irradiance in your postcode as well as other factors. This includes expected cloud cover based on meteorological data. The exact formula used to calculate this is a UK industry standard set by MCS. But, it is important to remember that this is just an estimate and if the weather has been particularly miserable over spring or summer months, as it has been recently, then your annual output is likely to be reduced. It is always best to think in terms of long term averages in solar, even on an annual basis weather patterns and cloud cover in one year may not match the next.
The MCS formula is usually on the conservative side, so it is more likely to underestimate than overestimate annual output. At Spirit we can augment the MCS method with our own models than can take into account localised shading such as trees or tall buildings, and generally our simulations give a higher and more accurate estimate of annual output.
An Example of How Cloud Cover Impacts a Solar System's Output
After having a PV system fitted, you will start to notice good years and bad years on your solar monitoring portal. In short, significant cloud cover will reduce your output in some months, but as you look over a longer period the differences should average out. As an example of this, let us take a look at Mark's solar output over the past few years. (You can view Mark's full experience of solar here.) It is a 4.2 kWp south facing system.
Below you can see the energy generated as shown on Mark's solar monitoring app for 2023 in green and the first five months of 2024 in orange.
As you can see, there is significant variance month to month and year on year, with February and May of 2024 producing significantly less energy than previous year, but March producing significantly more.
What is important, however, is that over the longer term 25 years the system will be operating for, these trends tend to even out. You can see this more clearly in the annual energy generation chart below. We have marked the estimated annual output of 4,242 kWh Mark was expecting from the MCS value in the quote in orange on the graph.
Does the Technology used in the Solar Array Matter?
There are a few technologies used in solar arrays that impact their output in cloudy skies. Solar panels have long been fitted with bypass diodes, which means if one solar cell stops working due to heavy shade the others around it will still produce electricity without being impacted by the shading on one particular cell. This has obviously beneficial effects when considering overhanging leafy trees, as well as cloud cover.
Another feature in some solar systems that can have an impact is the use of microinverters or optimisers underneath a solar panel. In contrast to a standard string inverter, as the name implies, the use of microinverters means each individual panel has its own small inverter that inverts the current from DC to AC at the panel level. This does have an advantage over standard string inverters where if the system is partially obscured by a cloud, the output of a particular panel is not dragged down by shade elsewhere on the array as can happen with string inverters.
These considerations, however, are only relevant where there are small passing clouds on an otherwise clear day. On heavily overcast days the irradiance will tend to be the same across the system. Some solar technologies such as Thin-film can increase efficiency in low light conditions as well as bifacial panels.
Ultimately the solar panel that boasts the highest efficiency in the laboratory are generally the best choice for all British weather, as the efficiency (which is a function of the percentage of irradiance on the panel that is translated to electricity) does not change in low light conditions.
Conclusion
In summary, solar panel output is significantly impacted by clouds and cloud cover, but this is very much to be expected and estimates of generation have a certain amount of cloudy days included in an annual kWh estimate. So do not worry too much if your generation dips during a spell of miserable weather, over the 25 year lifetime of a PV system you will still be quids in!
