Last reviewed: 4 September 2026. Yield figures are our own
rounded estimates built from the public 750–1,050 kWh/kWp range for the UK, and every assumption is
stated on this page. They are not MCS figures and should not be used for a warranty claim.
An 800 W plug-in kit in the UK generates somewhere between about 250 and 800 kWh a
year. That is a threefold spread, and the panel is barely involved in it. What decides the
number is where in the country you live, which way the panels face, how steeply they are tilted, what
shades them, and how much of the array’s DC capacity the 800 W inverter is allowed to pass through.
This page shows the whole calculation instead of quoting you one flattering figure.
The chain of multiplications
Solar output is not a single specification. It is one number multiplied by five factors, and
retailers tend to quote the first number and quietly assume the best case for the other five.
Start with your array’s DC capacity in kilowatt-peak. Two typical 430 W panels are 0.86 kWp.
Multiply by the annual yield for your region, then by an orientation factor, then by a mounting-angle
factor, then by a shading factor. That gives generation. What you actually save is generation
multiplied by the share you consume at the moment it is produced, because a self-installed kit cannot
be paid for exports. That last point is covered in
SEG payments and plug-in solar.
Annual yield by region
These are kilowatt-hours per kilowatt-peak per year, for a well-oriented, unshaded array. They are
our own rounded estimates from the published UK range, and we use exactly the same values in the
savings calculator, so the two never disagree.
| Region | kWh per kWp per year | 0.86 kWp array, ideal conditions |
|---|---|---|
| Scotland — north and islands | 780 | 671 kWh |
| Scotland — central and south | 830 | 714 kWh |
| North East / North West England | 850 | 731 kWh |
| Yorkshire & the Humber | 870 | 748 kWh |
| Northern Ireland | 870 | 748 kWh |
| Wales | 900 | 774 kWh |
| Midlands | 900 | 774 kWh |
| East of England | 950 | 817 kWh |
| London & South East | 970 | 834 kWh |
| South West England | 1,000 | 860 kWh |
The gap between the top and bottom of that table is only about 28%. Geography matters far less
than the next two factors, which is the opposite of what most people assume.
Orientation: the factor that halves your output
Facing south is the reference point. Everything else is a discount.
| Panels face | Factor | Effect on a 774 kWh Midlands array |
|---|---|---|
| South | 1.00 | 774 kWh |
| South-east or south-west | 0.95 | 735 kWh |
| East or west | 0.80 | 619 kWh |
| North-east or north-west | 0.60 | 464 kWh |
| North | 0.45 | 348 kWh |
A north-facing balcony loses more than half the output. This is the single most common reason a
kit disappoints, and it is knowable before you spend anything: stand where the panels would go and
check which way you are looking.
Mounting angle
A panel bolted flat against a vertical balcony rail is at 90°, and the sun in a British summer is
well above that. The penalty is real and it is worth planning around.
- Angled frame at roughly 30–40°: factor 1.00. This is the reference case.
- Fence or ground mount at a shallow angle: factor 0.95.
- Vertical, flat on a balcony rail: factor 0.70.
Vertical mounting is not all bad. It loses in June and gains a little in December, when the sun is
low and your consumption is high. If your interest is in cutting a winter evening bill rather than
maximising an annual total, the 0.70 understates it slightly. Details in
balcony solar panels in the UK.
Shading, and why a little is a lot
We apply a straightforward user-set factor: no shading 1.00, light 0.90, moderate 0.75, heavy
0.55. That is deliberately simple, and it errs on the generous side for one reason worth
understanding.
Solar cells in a panel are wired in series. A shadow across even one cell can drag down the string
far more than its share of the area, which is why bypass diodes exist and why a chimney shadow that
crosses the array for two hours costs more than two hours of output. If a neighbour’s tree clips the
corner of your balcony in the afternoon, treat that as moderate, not light.
The 800 W ceiling
The legal limit is on the inverter’s AC output, not on the panels. You may fit up to 2,000 W DC of
panels behind an 800 W AC microinverter — that DC figure comes from the DESNZ product specification
rather than from the statutory instrument itself, a distinction we set out in
the plug-in solar rules.
Oversizing the array is normal and sensible. A 1,600 W DC array behind an 800 W inverter spends
most of the year producing less than 800 W anyway, so the inverter is only the binding constraint for
a few hours around midday in high summer. Those clipped peaks are the hours when you are least likely
to be using the electricity at home, so in practice you lose less than the raw arithmetic suggests.
Our calculator applies a clipping factor rather than pretending the loss is zero.
Two worked examples
| Step | Best realistic case | Poor case |
|---|---|---|
| Array | 0.86 kWp, South West | 0.86 kWp, Scotland north |
| Regional yield | 860 kWh | 671 kWh |
| Orientation | South, ×1.00 → 860 | North-west, ×0.60 → 403 |
| Mounting | Angled frame, ×1.00 → 860 | Vertical rail, ×0.70 → 282 |
| Shading | None, ×1.00 → 860 | Moderate, ×0.75 → 211 |
| Generated per year | ≈ 860 kWh | ≈ 211 kWh |
Same kit, same money, four times the output. Before buying anything, run your own numbers in the
savings calculator and then read
is plug-in solar worth it, which turns kilowatt-hours
into pounds.
What the numbers do not include
Panels degrade, typically by a fraction of a percent a year, so year fifteen produces less than
year one. Dust and pollen cost a few percent unless you rinse the glass occasionally. And none of the
figures above are a guarantee: a cloudy summer is a cloudy summer, and a single year can land ten
percent either side of the average without anything being wrong.
The honest summary is that a plug-in kit is a modest, predictable appliance. It will not transform
a bill. Sited well, it will quietly remove two to four hundred kilowatt-hours a year from what you buy
from your supplier, for as long as it is bolted up.