Last reviewed: 4 September 2026, against version 2.0 of the DESNZ
Plug-in Solar Device Interim Product Specification, read in the original PDF. Output and money figures
use the same assumptions as our savings calculator and are our own estimates, not MCS figures.
The legal answer is up to four modules, with no more than two in series, up to 2,000 W of DC
panel capacity and no more than 120 V DC. The economic answer is smaller than that. Past
roughly 1 kWp the 800 W inverter starts throwing away your best summer afternoons, and past roughly
1.3 kWp the extra units land almost entirely in the half of your generation you give to the network for
nothing. Most British households are buying their fourth panel for something in the region of fifteen
pounds a year.
What the specification actually allows
Two documents govern this and they say different things, so it is worth being precise about which
limit comes from where.
- SI 2026/848 — the statutory instrument that made plug-in solar lawful in Great
Britain on 27 August 2026 — defines a plug-in microgenerator by its AC output: “a
maximum rated alternating current output not exceeding 800 watts”. It says nothing about how many
panels sit behind that inverter. It amends regulation 22(1)(c) of the Electricity Safety, Quality and
Continuity Regulations 2002, and regulation 3 extends to England, Wales and Scotland only. - The DESNZ Interim Product Specification (version 2.0) is where the DC limits
live. Clause 4.1 sets the electrical envelope: 800 VA AC output, a maximum output current of
3.5 A, and a maximum of 2,000 W of DC panel capacity. The specification also caps the array
at four modules, a maximum of two in series, 120 V DC, and requires the modules to
be rated at least IP55.
A 960 W figure also appears in clause 4.1 alongside the 800 VA and 3.5 A limits.
[VERIFICAR: exactly which quantity the 960 W value in clause 4.1 bounds — it is not obviously
the AC output ceiling, which is stated as 800 VA. Confirm before presenting it as a panel or inverter
limit.]
Two further constraints bite before the module count does. First, the whole kit must appear on the
ENA Type Test Register before it can be placed on the market (clauses 7 and 9.3), and
having submitted an application is explicitly not the same as complying — so you cannot mix and match a
compliant inverter with panels of your own choosing and still have a compliant device. Second,
Engineering Recommendation G98 currently permits one plug-in device per household,
even though the product specification itself is written around one device per final circuit. Until G98
is amended, one per household is the limit that applies. See
can I have two plug-in solar kits.
Why more panels stop paying: clipping, then export
Adding panels behind a fixed inverter costs you twice, and the second cost is much larger than the
first.
Clipping is the obvious one. On a clear June afternoon a 1.7 kWp array pointed south
will try to push well over 800 W through an inverter that cannot pass it, and the surplus simply never
becomes electricity. Our savings calculator models this as a 3% deduction from annual output for every
0.25 kWp above 1.0 kWp, capped at 12% — our own estimate of an effect that only bites on the
sunniest days. On an annual basis it is a modest penalty, which is why sellers of large arrays are
happy to talk about it.
Falling self-consumption is the one nobody mentions, and it is far bigger. A
self-installed kit cannot be registered for the Smart Export Guarantee, so every unit you do not use at
the instant it is produced is worth exactly nothing — see
does plug-in solar export to the grid. Your house
has a fairly fixed daytime baseline load. Doubling the array does not double the load, so the extra
generation lands disproportionately in the exported-for-free bucket. The bigger the array, the lower
the share of it that is worth money.
Four panel counts, worked through
Same house in every row: the Midlands, panels facing south on an angled frame, no shading, 430 W
modules. Generation and clipping use the
savings calculator exactly. The
self-consumption column is where we go beyond the calculator: it takes self-consumption as your input,
and we have applied a declining figure across the rows because that is what physically happens when you
enlarge an array in front of an unchanged household. Those declining percentages are
our own estimate, and if you disagree with them the calculator lets you set your
own.
| Array | DC capacity | Generation after clipping | Assumed self-consumption | Units actually used | Annual saving | Gained over previous row |
|---|---|---|---|---|---|---|
| 2 × 430 W | 0.86 kWp | 774 kWh | 70% | 542 kWh | £143 | — |
| 3 × 430 W | 1.29 kWp | 1,121 kWh | 60% | 672 kWh | £177 | £34 |
| 4 × 430 W | 1.72 kWp | 1,414 kWh | 52% | 735 kWh | £194 | £17 |
| 4 × 500 W (the legal ceiling) | 2.00 kWp | 1,584 kWh | 48% | 760 kWh | £200 | £6 |
Read the last column, not the one before it. The second panel in a kit is transformative. The third
is defensible. The fourth is a rounding error dressed up as an upgrade, and the difference between four
430 W modules and four 500 W modules is worth about the price of a takeaway per year.
Turn that into a buying rule. At a ten-year horizon — roughly the point where inverter and panel
warranties stop being a safe assumption — a third panel is worth paying up to about £340
for, and a fourth up to about £170, including whatever mounting hardware it needs. If
the quoted upgrade costs more than that, the money buys you nothing. We are deliberately not quoting
per-panel upgrade prices here: our own dated price survey covers complete kits, not single modules. See
how much a plug-in solar kit costs.
When a bigger array genuinely is the right call
The table above assumes an ideal south-facing angled mount. Oversizing earns its keep precisely
where the array is not ideal, because then it is rescuing a flattened output curve rather than
chasing a peak that gets clipped away.
- Vertical balcony mounting. Our mounting factor for a panel flat against a rail
is 0.70, and a vertical array almost never reaches 800 W AC at all. Clipping is close to
irrelevant, so extra capacity converts almost one-for-one into extra generation. - East–west split. Two panels facing east and two facing west never peak
together. You get a broad, low, double-humped curve that matches a household’s morning and evening
load much better than a single midday spike — and the morning and evening units are the ones you
are most likely to be at home to use. - Heavy shading. With a shading factor of 0.55 or 0.75 the inverter has plenty of
headroom, so extra panels are the only lever you have left. - Northern latitudes and winter weighting. If what you care about is having
something useful in October and February rather than a bigger July number, extra DC capacity helps
in exactly the months clipping cannot touch. See
plug-in solar in winter.
The physical limits that constrain your choice before economics does
Before you plan a four-module array, check three things that stop most people well short of the
legal ceiling.
- Weight and wind. Four modules is roughly 80–100 kg of glass and aluminium, plus
a wind load that no domestic balcony rail was designed to carry. This is the real limiting factor on
a flat, and it is not negotiable. See
mounting a plug-in solar panel safely. - Two in series, maximum. With a 120 V DC ceiling and a two-module series limit,
a four-module array is two strings of two. Your inverter must have two independent inputs; many
budget microinverters have one. Check the datasheet, not the marketing page. See
microinverters explained. - It has to be a registered kit. You cannot legally bolt two extra panels of your
own onto a two-panel kit and call the result compliant: the device on the ENA Type Test Register is
the specific combination that was tested. See
how to spot a non-compliant kit.
Our honest recommendation
For a garden, fence or flat-roof mount where you can angle the panels roughly south, buy
two panels, around 0.86–1.0 kWp. It sits exactly at the point where clipping is zero
and self-consumption is at its highest. For a vertical balcony, buy the largest
compliant array the structure can safely carry, because the mounting penalty gives you all the headroom
you need. Nobody should buy the fourth panel on a well-sited south-facing frame expecting it to pay for
itself.
Frequently asked questions
Can I add a panel to a kit I already own?
Only if the manufacturer sells that expansion as part of the registered device. Adding a module the
kit was not type-tested with takes you outside the specification, and outside the amended regulation 22
route that makes the installation lawful.
Does clipping damage the inverter?
No. A microinverter designed for a 2,000 W DC input simply holds its output at its ceiling and moves
the operating point up the panel’s voltage curve. It is lost energy, not a fault.
Do I have to notify my network operator differently for a bigger array?
No. The G98 notification is about the 800 W AC device, not the panel count, and it is due within 28
days of commissioning either way. See
how to fill in a G98 notification.
Would a battery fix the self-consumption problem?
Not inside the kit — SI 2026/848 explicitly excludes a device designed to import electricity in
order to store it for later supply. A separate home battery is a different purchase with its own
economics: see can I add a battery to plug-in
solar.
Is 2,000 W DC behind an 800 W inverter not absurd?
It is a generous headroom allowance, and it is deliberate: it lets manufacturers build kits for
vertical, shaded and northern installations where the nameplate figure will never be reached. On a
perfect south-facing frame it is far more capacity than the inverter can use.
Sources.
SI
2026/848, The Plugs and Sockets etc. (Safety) Regulations 1994 and Electricity Safety, Quality and
Continuity Regulations 2002 (Amendment) Regulations 2026 ·
DESNZ, Plug-in Solar Device Interim Product Specification
(v2.0) ·
DESNZ, plug-in solar consultation and government response, July 2026 ·
ESQCR 2002, regulation 22 (parallel operation) ·
Ofgem, price cap unit rates, 1 October – 31 December
2026.
Yield, orientation, mounting, shading, clipping and self-consumption factors are our own estimates,
listed in full on the savings calculator
and explained in our editorial methodology. Related:
plug-in solar kits ·
the rules ·
how much electricity plug-in solar
makes.