Inverters explained: string, micro and hybrid
The inverter is the component most likely to fail first, the one that decides which network process you are in, and the one nobody asks about. Three architectures, one threshold at 3.68 kW, and a trade-off called clipping that is not the fault it looks like.
Three architectures, and the number that decides your paperwork.
- String inverter — one box, all panels in series. Cheapest, simplest, and the whole string is affected by the weakest module.
- Microinverters — one small inverter per module. Handles shade and split roof planes; more devices on the roof.
- Optimisers with a string inverter — a middle route: module-level control, one central inverter.
- Hybrid — any of the above, plus battery charging on the DC side.
And the threshold: 3.68 kW registered capacity single-phase, 11.04 kW three-phase, is the limit of the simpler G98 process. Above it, you apply and wait.
The threshold that shapes the market
G98 covers fully type-tested micro-generators up to and including 16 A per phase, which the Energy Networks Association converts to 3.68 kW on a single-phase supply and 11.04 kW across three phases. Inside that, you connect and notify within 28 days. Outside it, G99 applies: apply, wait for assessment, then install.
This is why so many domestic inverters are rated at exactly 3.68 kW, and why quotes cluster just under 4 kWp.
The limit is on registered capacity, not panels. A 5 kWp array behind a 3.68 kW inverter has a registered capacity of 3.68 kW. G98 also allows a manufacturer to restrict a device’s rating in software, provided the settings are not accessible to the customer.
Clipping, and why it is usually fine
If the array can produce more than the inverter converts, the surplus is not generated — the output curve flattens at the inverter’s ceiling for those hours.
That sounds like waste and mostly is not:
UK arrays rarely reach nameplate. Rated output assumes Standard Test Conditions that a British roof reaches seldom and briefly.
A higher DC:AC ratio lifts the shoulders. More panels behind the same inverter means more output in winter, in cloud, and at the ends of the day — exactly when a household is more likely to use it.
The cost is a handful of summer afternoons, when generation is already exceeding what most homes can use and the surplus is exported cheaply anyway.
“What is the array’s kWp, and what is the inverter’s rated AC output?”
If the array is meaningfully larger, ask a second question: “how much output do you expect to be clipped in a year, and have you modelled it?”
A good installer has an answer and will explain the trade. An installer who has not considered it has sized by habit. And a system where the array is enormously larger than the inverter, with no explanation, is one where somebody is fitting cheap panels to a cheap inverter and hoping.
The same two numbers decide what you can add later — see upgrading a system.
When module-level electronics earn their cost
Microinverters and optimisers both solve the same problem: one weak module dragging down a string.
They are worth it when:
- part of the roof is shaded for meaningful periods — see shading
- the array is split across roof planes with different orientations or pitches
- module-level monitoring matters to you, which it does if you want to notice a failure
They are an unnecessary expense when the roof is a single clear plane. The honest test is whether the shading is real: our shading page covers how it is modelled and why a small shadow can cost more than its area suggests.
Hybrids and batteries
A hybrid inverter charges and discharges a battery on the DC side. The alternative is AC-coupling a separate battery inverter later.
If storage is likely, a hybrid now is usually cheaper than converting later — the battery sizing calculator covers what size actually cycles. But check two things: that the hybrid is compatible with batteries you would actually buy, and what its rated AC output is, because that is the number the network sees.
The datasheet
MIS 3002 requires the inverter datasheet as part of the minimum technical information before the contract is awarded. BS EN 50524:2021 governs what a photovoltaic inverter’s data sheet and nameplate must carry, and BS EN IEC 62109-1 covers converter safety.
What to look at:
- Rated AC output — the number that decides G98 or G99
- Maximum DC input and permitted DC:AC ratio — what array it can carry
- Number of MPP trackers — how many independent strings, which matters on split roofs
- Efficiency — European weighted, not just peak
- Warranty term, separately from the panels'
- Type test evidence for network connection
What we will not tell you
Which brand to buy. We have no independent UK reliability data, no test lab, and no commercial relationship with anyone — and a recommendation without evidence is just an opinion with a logo next to it. What we can tell you is which numbers to compare, and that the inverter is the component most likely to need replacing first.
Sources
- Engineering Recommendation G98, issue 2 The 16 A per phase threshold and its conversion to 3.68 kW. The ENA's own database requires registration; we read the freely available copy at this address.
- Plug-in Solar Device Interim Product Specification, version 2 Names BS EN 50524:2021 — data sheet and nameplate for photovoltaic inverters — and BS EN IEC 62109-1 for converter safety.
- MIS 3002: The Solar PV Standard, issue 6.0 Read locally. Requires the inverter datasheet as part of the minimum technical information.
Contains public sector information licensed under the Open Government Licence v3.0.
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