Rain can be good and heatwaves can be bad — this is how the different types of weather can affect the output of solar panels

With energy prices set to rise yet again in October, and solar panels becoming more readily available than ever before, it's no wonder more and more people are investing whether this kind of technology is right for their home.

For many the price of solar panels could be prohibitive, but with plug-in solar panels now legal in the UK, generating solar power could become a reasonable prospect for more and more homes.

But given the changeable weather we get here in the UK, how do the different weather types affect how well solar panels perform? I asked the pros to find out.

How does the sun affect solar panels?

As solar panels convert energy from the sun into power for your home (albeit at differing levels depending on whether you choose rooftop or plug-in solar panels), sunny weather is usually ideal for them. But the angle of the panels, as well as the time of year and the position of the sun will have an impact on how much power they generate.

'Between April and September, you can expect to generate around 65-75% of your annual solar electricity, with just 25-35% generated between October and March,' explains George Penny, Expert at The Solar Co.

'This is due to shorter days and the sun sitting lower in the sky, which reduces the amount of sunlight reaching your panels.

'Vertically mounted solar panels can also outperform traditional tilted systems in winter by up to 24.5%, making orientation a key consideration for year-round performance. However, even with optimised systems, winter output will always be lower than summer.'

And so it's best to look at the output of solar panels over a longer period of time. 'The practical takeaway is to think of a solar year as one long summer harvest with a quiet winter, rather than expecting a steady monthly figure,' suggests Andreas Pierrie, founder at Go Solar Today. 'Anyone judging their system on its December numbers is looking at the worst four weeks of the year.'

Are solar panels less effective in a heatwave?

While sunny days are good for solar panels, extreme heatwaves can have a negative impact, which may seem a bit baffling.

'It is one of the biggest misconceptions about solar power,' explains George Penny. 'Solar panels convert sunlight into electricity using semiconductor cells, and as these cells become hotter their electrical properties change, making it less efficient for electricity to flow.

'For every degree Celsius above the standard test temperature of 25ºC, solar panels typically lose around 0.3% to 0.5% of their power output.

'During a UK heatwave, panel surface temperatures can reach 65ºC or more, leading to noticeable efficiency losses.

'However, the abundant sunshine during a heatwave usually outweighs those losses, so while each panel produces slightly less, the sheer amount of sunlight means solar panels can continue generating large volumes of electricity.'

How does rain affect solar panels?

Rain itself might seem like the worst for solar panels, but it can actually have its benefits.

'Rain is the one people worry about and really shouldn't,' says Andreas. 'Panels are sealed units and are hail tested to withstand 25mm ice balls fired at around 50mph, so a British downpour is nothing. If anything rain helps, because it washes off the things that genuinely do cost you output: pollen in May, road dust, bird mess. Left alone, soiling can quietly shave a few percent off generation.'

But while the rain may do some of the work, you might still need to rope in the pros for a proper clean every so often. 'In fact, regular rain is often enough to keep panels clean in the UK, though some homeowners schedule professional cleanings once or twice a year to maintain peak performance,' says George.

How do clouds affect solar panels?

'The real ‘danger’ to solar output isn’t heat, it’s cloud cover, which can cut output by 75% or more on overcast days.' says George Penny. However, fear not, they won't stop working altogether. They'll just generate less than on a sunny day.

Having a battery as part of your solar panel set up can help you to store electricity generated on clear sunny days to use on days when it's a overcast, or on shorter days in winter.

How does snow affect solar panels?

While in most parts of the UK, snow is a something of a rarity, when it hits it can cause problems for your solar panels.

'If snow completely covers the panels, it can significantly reduce or temporarily stop generation because sunlight cannot reach the solar cells, explains Simon Bones, CEO of Genous. 'Fortunately, solar panels are normally installed at an angle, so snow will often slide off naturally as the panels warm up.'

Should you worry about the weather when it comes to solar panels?

While certain weather conditions can have a negative effect on your solar panels effectiveness, it's best to look at how well they perform over a longer term before deciding against them.

'Try not to become too obsessed with what your panels are doing hour by hour. Solar generation apps can become really addictive, but hourly generation doesn’t mean much,' explains Kian Milroy, Renewables Electrical Engineer at Heatable. 'The best way to judge efficiency is tracking production across the year.

'UK weather is incredibly variable, one miserable week in January tells you very little about whether your panels are performing properly.

'It’s also worth knowing that Britain is actually better suited to solar than people often assume. We may not have constant sunshine, but modern panels still generate from diffuse daylight. And when you add in that our relatively mild temperatures are good for panel efficiency, and our very long summer days give systems plenty of opportunity to produce electricity, it works out to be a fairly good match overall.

'If you’re considering solar, I’d be much more concerned about whether the system has been properly designed for your particular roof, rather than worrying about whether the British weather is ‘sunny enough’.'