Last reviewed: 5 September 2026. This page is arithmetic, not a
sales page. Kit prices are ranges published by UK retailers, not a price survey of our own, and we
label them as such. Electricity figures use the Ofgem price cap for 1 October to 31 December 2026:
26.32p per kWh. Generation figures are our own estimates from a public UK yield range.
The payback period on a plug-in solar kit is the purchase price divided by the annual saving, and
the annual saving is the electricity you use as it is generated — nothing else. There is no export
income, no grant, no tariff. That makes the sum unusually clean, and it also makes it unusually
unforgiving: a kit that pays back in five years and a kit that never pays back can be the same kit, in
two different houses.
Most coverage of this quotes a single number. A single number is not available, and anyone offering
one has quietly assumed your occupancy pattern.
The formula, in full
There are only four inputs, and you can populate all of them yourself.
- Kit price. What you actually paid, including any mounting frame, ballast or
fixings. Retailer-published prices for 800 W kits run from roughly £300 to £1,000, with two-panel
kits commonly around £700 to £800. - Annual generation. Panel capacity in kWp × yield. UK yield runs from about
750 kWh per kWp a year in the north of Scotland to about 1,050 on the south coast, averaging around
900. Deduct for anything other than an unshaded south-facing pitch: a vertical wall or balcony rail
loses a large share of that. - Self-consumption share. The proportion you use at the moment it is generated.
This is the variable that decides everything. - Unit rate. 26.32p per kWh at the current cap. Note that VAT on domestic
electricity is zero from 1 October 2026 to 31 March 2027 in Great Britain, which is already inside
the capped rate.
Annual saving = generation × self-consumption × unit rate. Payback in years = price ÷ annual saving.
That is the whole model. Everything below is that formula with numbers in it.
The scenario table
The table assumes a 0.9 kWp kit generating 800 kWh a year — a realistic figure for a well-sited but
not perfect installation in the middle of the country. Read down your price column and across your
occupancy row.
| Self-consumption | kWh used at home | Annual saving | £400 kit | £700 kit | £1,000 kit |
|---|---|---|---|---|---|
| 50% — empty most weekdays | 400 | £105 | 3.8 years | 6.7 years | 9.5 years |
| 70% — someone home much of the day | 560 | £147 | 2.7 years | 4.8 years | 6.8 years |
| 90% — home all day, loads deliberately shifted | 720 | £190 | 2.1 years | 3.7 years | 5.3 years |
Three things fall out of that grid immediately.
First, the spread is enormous: 2.1 years to 9.5 years for the same technology. Second, price and
behaviour matter about equally — moving from a £1,000 kit to a £400 kit roughly halves payback, and so
does moving from 50% to 90% self-consumption. Third, every cell in the table is inside the working life
of the panels, which is the genuinely good news about plug-in solar: even the worst cell pays back.
The DESNZ figure quoted at the launch of the plug-in regime is a saving of “up to £110 a year”. Our
50% row lands at £105, which is close enough to suggest the government figure assumes a fairly typical
occupancy and a kit around this size. Treat “up to” as doing real work in that sentence.
What the table deliberately leaves out
Simple payback flatters every energy purchase, and we would rather you knew how.
- Degradation. Panels lose output slowly — a fraction of a percent a year. Over a
five-year payback this moves the answer by weeks, not years, so we have left it out. Over a
twenty-five year lifetime assessment it matters more. - Electricity price movement. If unit rates rise, payback shortens; if they fall,
it lengthens. The cap changes quarterly. We do not forecast it, and neither should anyone selling
you a kit. - The cost of the money. If you would otherwise have left £700 in a savings
account earning interest, the true payback is longer. If you put it on a credit card carrying
interest, considerably longer. - Inverter replacement. The microinverter is the component most likely to fail
first. Nobody has a long enough field record for plug-in kits under the 2026 specification to say
what the real failure rate is. - Your own labour and any mounting costs beyond the kit — ballast blocks, a
ground frame, or a bracket that turns out not to fit.
[VERIFICAR: we have no primary source for microinverter failure rates in UK
plug-in service, because the compliant product category is only weeks old. We will add this when a
credible dataset exists rather than repeating a manufacturer’s MTBF claim.]
Where the honest arithmetic gets uncomfortable
Two situations turn a good purchase into a mediocre one, and both are common.
Vertical mounting
A panel bolted flat to a balcony rail or a fence faces the sky at 90 degrees rather than at the
30-to-40 degrees a solar panel wants in Britain. That costs a substantial share of annual output —
enough to move a five-year payback well past eight. If your only option is vertical, run the table
again with 500 or 550 kWh instead of 800 and see whether you still like the answer. See balcony solar panels.
Buying more panels than the inverter can pass
The specification allows up to 2,000 W of DC panels behind an 800 VA AC output, and there is a real
argument for a degree of oversizing: it lifts output in winter and in cloud. But every watt above the
inverter ceiling on a bright June day is clipped and lost, and the units you do gain arrive at midday,
which is exactly when you are most likely to be exporting them for nothing. Paying for capacity that is
clipped in summer and gifted in spring is the most common way to lengthen your own payback. See how many panels behind an 800 W inverter.
The lever nobody prices: your own routine
Compare two purchases. Spending £300 more on a bigger kit at 50% self-consumption buys you perhaps
£25 a year. Putting the dishwasher on a midday timer, moving the washing machine to lunchtime and
charging everything in daylight can move a household from 50% to 75% self-consumption, which on an
800 kWh kit is worth about £53 a year — and costs nothing at all.
| Action | One-off cost | Extra annual saving | Cost per £1 of annual saving |
|---|---|---|---|
| Add a third panel, 50% self-consumption | About £200 | About £26 | £7.70 |
| Move the kit from a vertical rail to a 35-degree frame | £60 – £150 for a frame | Substantial, site-dependent | Usually under £2 |
| Shift laundry and hot water into daylight | £0 | About £40 – £55 | £0 |
That last row is why using more of your solar at
home matters more than any purchasing decision you make after the kit is on the wall.
Work out your own number in ten minutes
- Get your half-hourly consumption data from your supplier. It is free and it is yours.
- Look at what you draw between 10am and 4pm on a weekday. That is roughly the ceiling on what a
plug-in kit can save you, because anything above it goes out of the building. - Estimate generation: kWp × the yield for your part of the country, then discount hard for
shading, orientation and any vertical mounting. - Multiply generation × self-consumption × 0.2632.
- Divide the kit price by that. Compare to how long you expect to live in the property — if you
are moving in two years, the kit is portable, which is a genuine advantage over rooftop solar.
Our savings calculator does the same sum with our
assumptions declared in full.
Frequently asked questions
Does the kit pay back faster if I have a battery?
A plug-in kit cannot include a battery — SI 2026/848 prohibits it. A separately installed battery
absorbs surplus, but it costs thousands and cannot be justified by plug-in solar alone. See can I add a battery to plug-in solar.
Can I fit two kits and halve the payback?
No. G98 permits one plug-in device per dwelling. See can
I have two plug-in solar kits.
Is a plug-in kit better value than rooftop solar?
Per pound spent, often yes; in absolute pounds saved, no, because rooftop systems are ten times the
size and can earn export income. They answer different questions. See plug-in solar versus rooftop solar.
Sources.
Ofgem, price cap unit rates and standing charges ·
SI
2026/848 ·
Ofgem, Smart Export Guarantee.
Kit prices are ranges published by UK retailers and labelled as such; generation and self-consumption
figures are our own estimates. All payback figures are our own arithmetic from the stated assumptions —
see editorial methodology. This is general information, not
financial advice.