Last reviewed: 4 September 2026. Requirements and figures below are from the DESNZ Interim Product Specification version 2.0 (July 2026) and SI 2026/848; standards are named as the specification names them.
In a plug-in solar kit the panel is the part you look at and the microinverter is the part that does the regulated work. Every number that makes a kit lawful is a property of the inverter, not of the glass: the 800 W AC ceiling in SI 2026/848, the 3.5 A current limit, the behaviour that makes the plug pins safe within 100 milliseconds of being pulled from the wall, the residual DC current the specification caps at 5 mA, and the type test registration on the ENA register without which the product may not be placed on the market. Two kits with identical panels can be one compliant and one not, and the difference is always the box behind them.
What it physically does
The panels produce direct current at a voltage that varies with light and temperature. The microinverter converts that into 230 V, 50 Hz alternating current and pushes it into the circuit its plug is in. It is a grid-following device: it takes its voltage and frequency reference from the mains and can only produce power while it can see them. That single fact explains most of its behaviour, including the one people find disappointing — a plug-in kit gives you nothing in a power cut, because the moment the grid disappears the inverter has nothing to follow and must shut down anyway.
Your appliances then draw from the nearest source. There is no clever routing: the inverter raises the voltage at its terminals fractionally above the supply, and current flows to whatever is consuming. If the house is using more than the panels are making, the shortfall comes from the grid and your meter records less import. If the house is using less, the surplus leaves the property — and with no export meter point and no Smart Export Guarantee tariff available to a self-installed kit, it is worth nothing. See does plug-in solar export to the grid and SEG and plug-in solar.
The regulated behaviour, requirement by requirement
| Requirement | The figure | Clause / standard | Why it matters |
|---|---|---|---|
| Maximum apparent power | ≤ 800 VA declared; the SI’s ceiling is 800 W AC | cl. 4.1, 6.4.1; SI 2026/848 definition | The one limit that is in the law itself |
| Maximum current at the point of connection | ≤ 3.5 A, under all test conditions | cl. 4.1, 6.4.1 | Tested at 0.94, 1.0 and 1.1 times the 230 V nominal supply, and to ≤ 3.5 A at reduced voltage of about 0.85 Un |
| Disconnection on loss of mains | Automatic disconnection within 100 ms | cl. 6.2.3.3.3, tested to BS EN IEC 62368-1 | Anti-islanding: it must not keep a dead network alive |
| Plug-pin voltage after unplugging | Below 34 V within 100 ms; capacitors above 100 nF discharged to ≤ 34 V within 1 s | cl. 6.2.3.3.3, figure taken from DIN VDE V 0126-95 | Why it is safe to pull a live generator out of a socket |
| Backup contact protection | Dangerous voltage removed within 2 s if the primary method fails | cl. 6.2.3.2 | A mechanical shutter or an independent electromechanical cut — a second line of defence |
| Residual DC current into the mains | ≤ 5 mA smooth DC, normal and fault conditions | cl. 5.7, per BS EN 62109-1 cl. 7.3.8 | Keeps the inverter from desensitising your RCD or RCBO |
| DC input limits | ≤ 2,000 W of modules, ≤ 4 modules, ≤ 2 in series per string, ≤ 120 V DC open circuit at the inputs | cl. 4.1, 4.2.2 | The 120 V limit exists to reduce the risk and consequences of DC arcing |
| Environmental rating | “Outdoor” category; IP55 or better outdoors | cl. 6.4.1 per BS EN 62109-1 Table 4; cl. 5.4 | Whether it may be mounted on the balcony or must come indoors |
| Fault reporting | An interface, on the device or via a phone, showing status, error messages and mitigation guidance | cl. 6.4.3, per BS EN IEC 62109-3 | Monitoring is a requirement, not a premium feature |
| Unused AC ports | Capped, removable only with a tool | cl. 6.4.2 | Stops a spare “parallel inverter” socket becoming an improvised second kit |
| Network compliance | Type tested to G98, registered and verified on the ENA Type Test Register before sale | cl. 7, 9.3 | The check you can do yourself before buying |
The standards it is built to
The specification does not write its own electrical engineering. It requires the inverter to conform to BS EN IEC 62109-1 for power converters in PV systems, 62109-2 for PV inverters, 62109-3 for module-level electronics where the module and inverter are supplied as an integrated unit, and BS EN 62116 for connection to the mains. Data sheets must follow BS EN 50524, with the applicable unit certificates under G98. Current limiting at reduced mains voltage is measured using the test procedure in BS EN 50438 and G98. Basic protection is required to BS EN 62109-1 clause 7.3.4 at decisive voltage class C, and where programmable electronic controls are relied on to meet a safety requirement they must satisfy Annex B of BS EN 62109-1 — that is, the software has to be safety-assessed too.
Compliance is verified by circuit analysis and fault simulation including single-fault conditions, and for Class I equipment a fault in the basic insulation between live parts and the protective conductor must not produce an unsafe condition.
Why 2,000 W of panel behind 800 W of inverter is not a mistake
It looks like an error and it is deliberate. A panel almost never produces its rated output: that figure is measured at standard test conditions — 1,000 W/m², 25 °C cell temperature — which a British roof or balcony meets for a handful of hours a year, and never on a hot day, because module output falls as the cells heat up. For most of the year the array delivers a fraction of its nameplate, so a larger array behind a fixed inverter lifts output in exactly the conditions where you need it: cloud, low winter sun, morning and evening. See plug-in solar in winter.
The price is clipping. On the brightest days the array can produce more than 800 VA and the inverter simply will not pass it; the surplus is never generated. With no export payment to lose, clipping costs a plug-in owner much less than it would cost a rooftop system — and the hours it clips are the sunny midday hours when a household is least likely to be using the power anyway. Our savings calculator applies a clipping factor above 1 kWp; the assumption is declared on the page as our own estimate, not a manufacturer figure. See also how many panels behind an 800 W inverter.
What the inverter cannot do
- It cannot run your house in a power cut. Grid-following means exactly that, and the anti-islanding requirement makes it mandatory.
- It cannot charge a battery. The SI excludes devices designed to import electricity to store it, and the specification requires the instructions to carry a prominent warning that the device is not intended to be used with a battery energy storage system. See can I add a battery to plug-in solar.
- It cannot be paralleled with a second one to reach 1,600 W. Any unused AC connection must be capped and only removable with a tool, and G98 currently allows one device per household. See can I have two plug-in solar kits.
- It cannot compensate for a bad connection. Push 3.5 A through metres of thin flexible cable and the voltage at its own terminals rises until its G98 protection settings trip it out. That is one of several reasons extension leads are prohibited — see extension leads and plug-in solar.
Buying and living with one
Three practical points, all of them derived from the specification rather than from marketing.
- Check the model on the ENA register, and match the register entry to the model number on the product’s permanent marking, not to the listing title. See how to spot a non-compliant kit.
- Decide indoors or outdoors before you buy. The documentation must state whether the inverter is designed to be installed outside, inside, or either. An outdoor-mounted inverter needs IP55, and its socket does too.
- Never break a DC connection under load. The specification requires the instructions to say that DC connectors must not be disconnected under load and that the AC supply must be switched off first. Breaking a DC circuit under load is how you draw an arc, and it is the reason for the 120 V ceiling. See cleaning and maintenance and monitoring a plug-in solar kit.
Frequently asked questions
Microinverter or string inverter?
For plug-in solar the distinction barely arises: with at most four modules and at most two in series, the device the specification describes is a small module-level or two-port inverter. Large string inverters belong to certified rooftop systems — see plug-in solar vs rooftop solar.
Can I replace the inverter with a bigger one?
No. The kit is type tested and registered as a unit, and the specification requires that the product is not modified or installed in a manner not specified by the manufacturer. A different inverter is a different device, and an unregistered one.
Does it make a noise, or interfere with the radio?
It should not. Electromagnetic compatibility is tested to BS EN IEC 61000-6-1 for immunity and 61000-6-3 for emissions, at maximum power across the declared DC input range.
How long will it last?
Longer than the warranty, usually, but the inverter is the part with the electrolytics and the electronics, so it is the part that fails first in most PV systems. Ask for the warranty term on the inverter specifically rather than the headline “25 years” that refers to the panel. See warranty and your UK rights.
Does it need earthing or an isolator?
Not as a separate installation task — the plug and the existing circuit provide the connection, and the specification’s protection scheme is built around that. Anything else is fixed-wiring work for a competent electrician working to BS 7671. See do I need an electrician.
Primary sources.
DESNZ, Plug-in Solar Device Interim Product Specification version 2.0, July 2026 (PDF) ·
SI 2026/848 (legislation.gov.uk) ·
ENA Type Test Register ·
ENA, G98 and G99 documents and forms ·
Ofgem, Smart Export Guarantee.
General information, not electrical advice — see our editorial methodology.