Do Solar Panels Need Direct Sunlight? How UK Weather Affects Output

Do solar panels need direct sunlight to work?

If you live in the United Kingdom and are considering having solar panels installed on your roof for your energy needs, you may have wondered whether they are even worth it, especially as the weather here is often overcast or rainy. The good news is that direct sunlight isn’t needed for your solar panels to generate electricity.

Even when it’s cloudy outside, sunlight still gets through, meaning your panels will still work.

Some of the confusion comes from when people mix up solar thermal systems with photovoltaic (PV) panels. In other words, solar panels don’t need heat for them to work. PV panels are responsible for converting light into electricity, whereas solar thermal technology relies on the sun’s energy to heat water.

The total output of your solar panels can vary, and it largely depends on the time of the year, the strength of the available light, what time of the year it is, shading, and which direction your roof faces.

If you’re thinking about having solar panels installed, it’s best to understand these factors because they can help you work out how much electricity solar panels generate throughout the course of a typical year in the UK.

Direct sunlight vs ambient daylight

No, for your solar panels to generate electricity, they don’t need to be in direct, blinding sunlight, and instead of relying on the sun’s heat, they generate electricity from sunlight.

Light energy is converted into electrical energy by the semiconductor cells used in photovoltaic panels. Electrical charges that can be used to produce usable energy for your home occur when sunlight reaches the cells.

This process works with both direct sunlight and filtered daylight (also known as diffuse light) when it’s cloudy. Diffuse light is a common term that refers to sunlight that has been scattered by clouds and the atmosphere before reaching the ground.

That means your panels are still perfectly capable of generating electricity when the weather is dismal, grey, and overcast. However, the amount of electricity that solar panels can generate is typically far greater when the sun shines directly onto them, but it’s important to remember that they won’t stop working for you just because it is cloudy.

In contrast, solar thermal systems work in a different way. Rather than generate electricity, the purpose of these is to provide heat to heat water or other fluids using solar energy. Although both technologies benefit from sunlight, they have completely different jobs.

Does the UK get enough sunshine for solar panels?

Yes. Solar PV is a practical option for many homeowners because the United Kingdom receives enough solar energy and daylight. In other words, you don’t have to live in a country where it’s always hot and sunny to reap the many benefits of solar technology.

Although the exact figures vary somewhat depending on where you live, the Met Office‘s 1991–2020 climate averages record approximately 1,403 hours of sunshine a year across the UK. It’s also worth noting that sunshine hours do not represent every hour of daylight available to solar panels. However, these figures act as a useful guide for putting the UK climate into perspective.

Instead of assuming that each day will be sunny no matter where you live, a suitable solar panel system is designed around roughly how much of a resource solar can be for your property. Some of the key factors that can determine how efficient your system will be include your typical household electricity use, shading, the direction your roof faces, and its pitch.

Can solar panels work in the shade or on cloudy days?

Yes. Solar panels work in cloudy weather and in some shaded conditions, but exactly how much electricity they will generate also depends on other factors, such as how well your system has been designed and what obstacles or features may be blocking your panels from direct sunlight.

How well do solar panels work on cloudy days?

Throughout the day, clouds scatter, which can significantly affect how much sunlight reaches your panels, but they won’t entirely block the light. Some light is still able to pass through the clouds, meaning your solar cells will keep doing their job.

You may find that your system’s rated peak output capacity could dip to around 10%-25% when it’s overcast, for example. This isn’t an exact figure, and should be viewed more as a rough guideline. The actual results will depend on how thick the clouds are, where the sun is positioned, what time of year it is, and how well-designed and installed your system is.

For example, a system that’s typically capable of reaching 4kW (when there are no clouds in the sky) might only be able to produce 1kW or less when there’s heavy cloud. However, this also doesn’t mean that it will produce this same low amount each time there’s heavy cloud, or that it will operate at the same low level for the remainder of the day.

It’s important to distinguish that no output is not the same as lower output. Over the course of an entire year, and despite the constantly changing weather in the United Kingdom, solar panels can still generate plenty of useful amounts of electricity. 

Does rain affect solar panels?

The other good news is that your solar panels won’t stop producing energy for you just because it’s pouring down outside.  They have been designed to work well even when it’s raining, although output will likely be reduced when there’s a combination of heavy cloud cover and rainfall.

One good thing about the rain is that it can help clean up your solar panels by washing away surface dirt and loose dust, but you may still need your solar panels occasionally inspected and cleaned because the rain might not wash away more stubborn things like leaves and bird droppings or various other deposits.

You also have to consider the temperature outside. Solar panels may not be so effective if their cells get too hot. The most favourable operating conditions are on days when it cool, bright, and sunny days with no clouds.

While the amount of available sunlight that hits your panels remains a key influence, it also doesn’t mean that cooler weather helps to generate even more electricity.

What about solar panels in winter?

Your solar panels will work throughout the year, even in winter, but you may find that the amount of energy they produce is typically lower compared to during the summer months.

Therefore, the changes in the season, especially from summer to winter and autumn, should be considered a key part of your solar installation planning. Instead of gauging what your system can produce on a single afternoon in the winter, a better way to assess it should be on the entire year’s expected generation.

You can learn more about this by reading our solar panels in winter guide.

Dealing with heavy roof shading (trees and chimneys)

It’s also important to distinguish hard shading and cloud cover. For instance, a chimney, large tree with overhanging branches or nearby building can cast a concentrated cloud over a section of your roof panels, whereas clouds reduce the light reaching a wider area.

In some cases, it might be impossible to avoid certain types of shading, but the effect of that shade largely depends on what time of the day the shadow falls, the technology used in the installation, and how much of the array is covered in shade.

How does shading affect solar panel output?

Solar panels in traditional systems that use strong converters are connected together in a series, and depending on the type of shading and the equipment being used, a shaded panel can limit the output of other panels connected to the same string.

Many people compare this to a set of Christmas tree lights. For example, if one part of the circuit is having an issue, it can affect more than the individual component. Modern panels come with bypass diodes which are capable of reducing some shading losses, so it’s not an exact comparison, but the underlying principle helps to paint a better picture as to why roof layout matters.

As mentioned, large trees with overhanging branches that cast a shadow on your roof, or chimneys/dormer windows that might also cast a shadow on your panels at certain times of the day, may also present a small problem when it comes to how much electricity your panels produce.

Can optimisers or microinverters help?

Yes. To help reduce the impact of partial shading, you can look to microinverters or DC power optimisers, but exactly which of these you will need depends on the roof layout and shading pattern.

Microinverters are designed to convert the electricity produced by individual panels into usable AC electricity, whereas power optimisers work with individual panels to manage their output. Both technologies enable the rest of an array to continue generating electricity efficiently even if one panel is in the shade.

However, they don’t eliminate the shade or recover the sunlight that a panel can’t receive. If, for example, a nearby building window casts substantial shade over a large section of your roof, less electricity will be generated by the panels in the shade. The benefit is that the effect may be limited, rather than unnecessarily reducing the output of other panels.

The most suitable system for your roof depends on the type of property you have. You may personally find that the most viable solution for your situation is to position the array away from shaded areas.

For others, it might be worth considering panel-level electronics. A proper assessment is the best way to determine which option is more relevant and whether you will need them or not, instead of them simply being added to every installation.

At Spartek, careful system design helps identify potential shading issues and assess the most suitable layout and equipment for your property.

The intermittency problem: compensating for weak sunlight

The main limitation of solar power technology isn’t that it just stops working each time a cloud shows up, or a bit of shade covers one or more of your panels. It is that generation output varies somewhat throughout the day and year. Plus, solar panels aren’t capable of generating electricity when it’s dark.

Even when it’s sunny, electricity generation will increase and decrease as the sun moves across the sky. Throughout the winter, when days are much shorter in terms of how much light there is, it tends to mean solar systems will generate less usable electricity than in summer. 

To compensate for cloudy weather, is it worth installing more solar panels?

The best thing to do is base your system sizing on how much electricity your home generally uses. Other key factors to consider are how much space there is on your roof available, expected generation, and your financial budget.

Some homes may find it more beneficial to install systems with slightly more capacity than their current annual usage alone suggests, especially if they plan on adding things like a heat pump or an EV at some point in the future.

In certain situations, it might also be worth including a 20% buffer as part of your initial planning guideline, but you don’t have to, and it still doesn’t guarantee that your system will achieve a particular output expectancy level during the winter when it’s cloudy.

With that said, adding extra panels won’t fully compensate you when there’s hardly any sunlight over prolonged periods, plus the cost of an oversized system may work out more than the additional electricity is actually worth.

A professional installer should estimate annual generation for your specific roof and clarify how the system will most likely perform in spring, summer, autumn, and winter. The aim is to find a workable balance between how much electricity your household can use, solar generation, and the installation costs.

Can a solar battery help on cloudy days and at night?

One highly effective way of helping you make better use of your electricity is a solar battery. When the solar panels generate more electricity than you need, any surplus electricity they produce can charge your battery so it can be used later on.

With a solar battery, it’s then possible to use the energy that has been stored during periods when your panels are generating less, such as throughout the night. However, batteries aren’t capable of storing energy that hasn’t been produced by your panels, so their practicality solely depends on the amount of surplus electricity that is generated to charge it.

When systems have solar panels and battery storage, they are highly effective, but the ideal size of the battery you will need will depend on your budget, the size of your solar array, and your typical energy requirements.