Plug-in solar is sold on a simple promise: buy a kit, plug it in, watch the bills fall. Whether that adds up to a sensible purchase depends on numbers that most sellers do not put in front of you — chiefly how your panels are mounted, and how much of what they generate you are actually at home to use.
This calculator works out both, and shows you the seasonal shape of what you would generate.
Free tool
Plug-in solar payback calculator
A balcony kit only saves you money on the electricity you use while it's being generated. This works out both — and shows you how little you'll get in a British December.
Pays for itself in
Saved per year
£85
20-year total
£1,618
Yearly output
444 kWh
Share you'd use
74%
What you'd generate each month
How this calculator works
Annual output starts from typical UK irradiance for your region — roughly 780 kWh to 950 kWh per kilowatt of panels per year, in line with PVGIS modelling — then adjusts for orientation, mounting angle and shading. Vertical railing mounting is the big one: it typically costs around 30% of the annual total against a properly angled frame, because the panels are badly aimed at the high summer sun.
Vertical panels claw some of that back in winter. The low December sun strikes an upright panel much more squarely, so the seasonal curve is flatter than a roof array's — which is why the winter bars here don't collapse as far as you'd expect.
The savings figure only counts electricity you actually use. Plug-in kits feed the ring main in your flat or house. Anything generated while nothing is switched on flows out to the grid, and with a plug-in kit you are almost certainly not being paid for it. A small kit in a home that's occupied all day is worth far more per watt than a big kit in an empty one.
The 20-year figure assumes output falls by about 0.5% a year and that electricity prices stay flat in real terms. It excludes replacing the microinverter, which typically carries a 10–12 year warranty. If electricity gets more expensive — as it consistently has — you will do better than the figure shown.
The default £699 kit price reflects a complete 800 W balcony setup including panels, microinverter and railing mounting hardware. Cheaper panel-only bundles exist, but the mounting is rarely included.
Estimates for guidance only — not a quote or a performance guarantee. Real output varies year to year with the weather. Check your kit's certification and any notification duties to your network operator before buying.
What matters most
Two inputs dominate the result, and neither is the one people expect.
How the panels are mounted. Hanging flat against a balcony railing is convenient and landlord-friendly, but it aims the panels badly at the high summer sun — precisely when most of the year’s generation happens. Against an angled frame it typically costs around 30% of the annual total. Winter is the consolation: an upright panel meets the low December sun more squarely, so the seasonal curve is flatter than a roof array’s.
Whether anyone is home. A plug-in kit feeds your ring main. Electricity generated while the flat is empty flows out to the grid, and with a plug-in kit you are almost certainly not paid for it. This is the counterintuitive part: a smaller kit in an occupied home can pay back faster than a bigger one in an empty house, because a larger kit simply gives more away.
Reading the result
The payback figure is kit cost divided by annual saving, where the saving counts only electricity you use as it is generated. The 20-year total assumes output declines by roughly 0.5% a year and that electricity prices hold flat in real terms — a conservative assumption, given how prices have actually moved.
If your result comes back above 40 years, the combination you have entered does not pay back within the panels’ working life. A sunnier aspect, an angled mount, or a smaller kit will all help.
Frequently Asked Questions
How long does a plug-in solar panel take to pay for itself in the UK?
For a typical 800W balcony kit costing around £699, payback usually lands somewhere between 7 and 12 years. The spread is wide because it depends heavily on how much of the electricity you use as it is generated. A kit in a home occupied during the day can pay back several years sooner than the same kit in a house that is empty from morning to evening.
Why does vertical railing mounting reduce output so much?
A panel hanging flat against a balcony railing is badly aimed at the high summer sun, which is when most of the year's generation happens. Against a properly angled frame at about 30 degrees, vertical mounting typically costs around 30% of the annual total. It does claw some of that back in winter, when the low sun strikes an upright panel more squarely.
Do I get paid for the solar electricity I do not use?
With a plug-in kit, almost certainly not. These kits feed the ring main in your home, and anything generated while nothing is switched on flows out to the grid unpaid. This is why the calculator only counts electricity you actually use when working out savings, and why a smaller kit in an occupied home often pays back faster than a larger kit in an empty one.
How much electricity will a balcony solar kit generate in winter?
Far less than in summer. A December in the UK typically yields somewhere around a fifth to a tenth of what June produces, depending on where you live and how the panels are mounted. Vertical railing-mounted panels hold up comparatively better in winter than an angled array, because the low winter sun hits them more directly.
Are the calculator's figures a guarantee?
No. They are estimates for guidance only, based on typical UK irradiance figures in line with PVGIS modelling. Real output varies year to year with the weather, and the 20-year total assumes panel output falls by about 0.5% a year with electricity prices flat in real terms. Treat the result as a realistic ballpark, not a quote.