How much does a 400W panel generate in a year?

As a rough guide, a single well-placed 400W panel might generate around 300–400 kWh a year in the UK. The actual figure depends on location, orientation, tilt and shading, so treat it as indicative. Use the calculator for an estimate at your postcode.

Why does the output change so much month to month?

The amount of sunlight reaching a panel varies hugely across the year. The summer months produce the most, while December and January produce only a small fraction of that, because the days are shorter and the sun is lower. A yearly total smooths these peaks and troughs.

What reduces a 400W panel's output?

Shading from trees, buildings or chimneys is the biggest avoidable factor, followed by a poor orientation or tilt. Dirt on the panel, high temperatures and simply being further north all reduce output too.

Does a 400W panel cut my bill by the cost of all it generates?

No. Only the electricity you use while it is being generated reduces your bill. Anything you do not use at the time is exported, which may currently earn nothing for a self-installed plug-in system.

Guide

How much does a 400W solar panel generate?

What a single 400W panel might realistically produce in the UK, how output varies through the year, and what reduces it.

Written and edited by Christopher Panteli

Last reviewed 19 June 2026

Christopher is the founder and editor of MyPlugInSolar. He oversees the site’s research standards, data tools and editorial process. He is not an electrician or solar installer, and specialist technical claims are sourced from official documentation or reviewed by appropriately qualified professionals.

A 400W panel is a typical single-panel size, so it is a helpful unit for thinking about small solar setups. As a rough guide, a well-placed 400W panel might generate around 300–400 kWh a year in the UK. That is indicative rather than fixed: your own result depends on where you live, which way the panel faces, its tilt and any shading.

One important note before the numbers: plug-in solar is not currently legal to sell, supply or use in the UK, with a Government consultation open until 30 June 2026. See the legal status for the detail.

What a 400W panel might generate

For a well-placed 400W panel in the UK, an annual figure of around 300–400 kWh is a reasonable expectation. The lower end reflects less favourable conditions — a more northern location, a poorer orientation, or some shading — and the upper end reflects a sunny spot, a good south-facing angle and little or no shade. As a rule of thumb, this is roughly half what you might expect from an 800W system, which is what you would expect from doubling the panel area.

Because the figure is so site-specific, the most reliable number is one calculated for your address. Our calculator uses PVGIS data from the European Commission's Joint Research Centre to estimate output for your postcode, orientation and tilt.

How output varies through the year

A yearly total hides a lot of variation. UK solar output is strongly seasonal: the long, high-sun days of summer produce far more than the short, low-sun days of midwinter. December and January are the weakest months by some margin, while the months around midsummer do most of the annual work. A panel that comfortably covers your daytime background use in June may contribute only modestly in the depths of winter.

Indicative only — the pattern of monthly variation, not exact figures, which depend on your site.
Time of year	Relative output	What to expect
Summer (around June)	Highest	Long days and high sun — the strongest months of the year.
Spring and autumn	Moderate	Useful output that tapers as the days shorten.
Winter (December–January)	Lowest	A small fraction of summer; short days and low sun.

Our guide on plug-in solar in winter looks at the darker months in more detail and how self-consumption still helps.

What reduces it

Several things can pull a panel below its potential:

Shading. Trees, chimneys, neighbouring buildings or a rail above the panel can cut output noticeably, especially around the middle of the day.
Orientation and tilt. A position that faces away from south, or a flat or awkward angle, captures less light over the year.
Location. The further north you are, the less annual sunlight reaches the panel.
Dirt and temperature. A dirty surface blocks light, and very hot panels are slightly less efficient.

Generation is not the same as savings

Whatever a 400W panel generates, only the electricity you use while it is being generated reduces your bill. A single panel is well suited to offsetting steady daytime loads like a fridge or router. Anything you do not use at the time is exported, and for a self-installed plug-in system that export may currently earn nothing, depending on schemes such as Ofgem's Smart Export Guarantee. For a figure tailored to your home, put your details into the calculator.

Frequently asked questions

How much does a 400W panel generate in a year?: As a rough guide, a single well-placed 400W panel might generate around 300–400 kWh a year in the UK. The actual figure depends on location, orientation, tilt and shading, so treat it as indicative. Use the calculator for an estimate at your postcode.
Why does the output change so much month to month?: The amount of sunlight reaching a panel varies hugely across the year. The summer months produce the most, while December and January produce only a small fraction of that, because the days are shorter and the sun is lower. A yearly total smooths these peaks and troughs.
What reduces a 400W panel's output?: Shading from trees, buildings or chimneys is the biggest avoidable factor, followed by a poor orientation or tilt. Dirt on the panel, high temperatures and simply being further north all reduce output too.
Does a 400W panel cut my bill by the cost of all it generates?: No. Only the electricity you use while it is being generated reduces your bill. Anything you do not use at the time is exported, which may currently earn nothing for a self-installed plug-in system.

Sources

1. PVGIS photovoltaic geographical information system — European Commission Joint Research Centre
2. Smart Export Guarantee (SEG) — Ofgem

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