Does plug-in solar work in winter?

Does plug-in solar work in winter?

Yes. Solar panels need daylight, not heat. A cold, bright January day can be a perfectly good day for them.

What changes in winter is how much daylight there is. For most of December and January, plug-in solar gives a small top-up to the things that are always on in your home. It won’t run the house.

Why does solar produce less electricity in winter?

It’s the time of year, not the temperature. Four things work against the panels:

• Shorter days. In our Manchester example, the panels make electricity from about 9am to 3pm on an average December day. In June, it’s from about 6am to 8pm.
• A lower sun. At midday in late December, the sun is only about 13° above the horizon in Manchester, against about 60° in June. Low sunshine is weaker by the time it reaches the panels.
• Greyer weather. Winter has more dull days, and cloud cuts output sharply.
• Longer shadows. A low sun casts long shadows, so something that doesn’t shade the panels in summer can shade them for much of a winter’s day.

Put together, south-facing panels in our Manchester example get less than a quarter as much solar energy in December as they do in June.

Does cold weather stop solar panels working?

No. Cold weather isn’t what causes the big winter drop.

Solar panels can actually work slightly more efficiently when they’re cool. But that small effect is swamped by how little solar energy there is in a UK winter. Shorter days, a lower sun and the weather matter far more.

Frost and snow are different. Anything covering the panels blocks the light until it clears.

How much can an 800W plug-in solar system generate in winter?

First, what “800W” means

There are two different numbers here.

The UK rules limit a plug-in solar inverter to 800VA of apparent power supplied to your home, averaged over 10 minutes. In normal use that effectively caps real power at 800W.

That is separate from the panels themselves. A compliant kit can have more than 800W of solar panels, up to 2,000W under the current specification.

For the generation examples on this page, we’ve modelled 800W of solar panels. So when we say they make 0.7 kWh on an average December day, we mean 800W of panels, not that every kit sold as an “800W kit” will produce exactly the same amount.

Watts (W) tell you how much power the kit is producing right now. Kilowatt-hours (kWh) tell you how much electricity it has made over time, and it’s the same unit as on your electricity bill. Even running flat out at 800W for a whole hour, a kit would make 0.8 kWh.

In winter, the panels don’t get close to their rated output. In our Manchester example, the best hour of an average December day averaged about 150W. Even the brightest December hour in 19 years of weather data stayed under 500W.

Month by month

Here’s what 800W of panels makes on an average day in each month in our Manchester example, in kWh. The first column is panels tilted on a frame, as you might have in a garden. The second is panels fixed upright on a balcony railing or wall. Both face south.

Modelled example: 800W of panels in Manchester, facing south, no shade. kWh on an average day.

Don’t try to memorise the numbers. Look at the shape of the year. On the tilted frame, output climbs quickly through spring, stays high from April to July, then falls away through autumn to its lowest in December and January. Upright panels make much less in summer, but a little more in midwinter.

Over the whole of December, the tilted frame makes about 23 kWh, against about 89 kWh in June. November to February together bring in about 15% of its year.

Where you live makes a difference too. In December, the same frame makes about 0.4 kWh a day in Inverness and about 0.9 kWh in London. For yearly figures for nine places across Great Britain, see how much electricity 800W plug-in solar generates.

Some winter days are far better than others

An average hides a lot. Here’s how much the tilted frame made on different kinds of December day in our Manchester example, across 19 years of weather:

The typical day is below the 0.7 kWh average because a handful of bright days pull the average up.

So winter solar isn’t zero. It’s much lower, and it varies a lot more.

Do solar panels still work on cloudy days?

Yes. Daylight still reaches the panels through cloud, so they keep making electricity. But the brighter it is, the more they make, so a grey day produces much less.

That’s why winter output swings so much from one day to the next, as the December figures above show. A run of grey days doesn’t mean something’s wrong with your system. In a UK winter, it’s normal.

Does panel position matter more in winter?

Yes. The winter sun stays low in the southern sky, so buildings, fences and trees cast much longer shadows. A spot that’s sunny in June can spend much of a December day in shade.

Direction matters more too. Across the nine Great Britain locations in our 800W generation guide, panels split between east and west produced roughly 0.1 to 0.4 kWh on an average December day. In Manchester, the figure was about 0.3 kWh. That’s because the winter sun rises and sets so far round to the south. Upright, south-facing panels catch the low sun well, which is why they beat the tilted frame in midwinter.

Before you buy, check what shades your chosen spot around the middle of a winter’s day, not just in summer. For how direction, angle and shade affect the whole year, see our 800W generation guide.

Is plug-in solar still worth having if winter output is low?

Judging it on December alone is misleading. A plug-in kit works all year, and winter is the weakest part of it. Most of its electricity arrives between spring and early autumn.

What you make in winter still counts when you use it, for example by covering some of what’s always running, like the fridge and router. You can’t store it for the evening, because batteries aren’t part of the plug-in rules yet.

Whether it makes sense for your home is a bigger question. Our guide Is plug-in solar worth it in the UK? covers it.

What will WattVerdict test?

Everything on this page comes from a model: established weather data run through the European Commission’s solar tool. We haven’t measured a winter yet.

WattVerdict’s own testing is designed to show what systems actually do in changing UK conditions. Each test runs for 14 days and measures the electricity a system delivers, alongside a measurement of the sunlight available at the time. That lets us put a dull winter fortnight in context next to a bright summer one. When we have winter results, we’ll add them here.

How we test explains the method, and Live testing is where results will appear.