New build solar: is the developer's system any good?

What the standard actually guarantees

An output target, and that the electricity reaches the household. That is the whole of it.

It does not require, and a compliant house may lack:

• the best roof plane — the target is benchmarked against a south-east to south-west array, but the panels may be sited elsewhere provided the modelled output is met
• a battery, or any wiring toward one
• inverter headroom for future expansion
• monitoring you can actually see
• MCS certification — which is what your export tariff will ask for
• any relationship to your consumption — see self-consumption

None of that is a scandal. It is what happens when a rule sets a floor: the floor is what gets built, most of the time, by most builders.

The inverter number worth knowing

This is the single most useful thing on this page, and almost nobody asks about it.

G98 — the simpler network process, where you connect and notify afterwards — applies up to 16 A per phase, which the Energy Networks Association’s own document converts to 3.68 kW single-phase. Above that, the installation falls under G99: apply first, wait for permission.

Now look at the 40% formula. A house with a 50 m² ground floor has a target of 4.4 kWp. A 60 m² ground floor gives 5.28 kWp. Those are comfortably above 3.68 kW.

So on many new builds the panel capacity exceeds the G98 threshold while the registered capacity — the inverter — need not. An installer can fit a larger array behind a 3.68 kW inverter, or limit it in software, and stay in the simpler process.

Clipping is not automatically bad — a slightly oversized array behind a smaller inverter is a legitimate design that lifts winter and cloudy-day output. What matters is knowing which you have, and what it means for upgrading later.

What developers themselves said about the rules

The consultation response is unusually candid about the industry’s objections, and they tell you where the compromises in your house will be.

Respondents — “predominantly … builders/developers and manufacturing/supply chain” — argued that the proposed solar coverage was “not achievable in practice for certain house or roof types e.g. houses with dormer windows”, and raised concerns about “the challenging impact of the notional building assuming south-facing solar panels”.

Others raised grid connection capacity, “specifically … the difficulty of obtaining connections for export capacity”, and warned that sites already loaded with gas infrastructure might need upgraded electricity connections.

Read that as a buyer and you get a practical checklist. Dormers and complex roofs are where provision gets squeezed. Non-south roofs are where output falls short of the notional case. Export capacity is where a large site may disappoint — the government’s impact assessment concedes there may be sites where export “is not possible with current infrastructure”, though it expects that to be rare.

Batteries were asked for, and not required

Respondents advocated including battery storage in the standards, partly to reduce pressure on the grid. It was not adopted, and Approved Document L addresses storage only where it is installed — noting that fire safety and electrical requirements under Approved Documents B and P apply, and that systems should be “an appropriate size for the site”.

The consequence for you is straightforward. Solar without storage pays according to how much you use while the sun is shining. On a house that is empty from eight until six, that is the weakest case for solar there is — which is why self-consumption is the number that decides the value, not the kWp on the roof.

If a battery is likely, the cheapest moment is before plasterboard.

What a good system looks like

Against a minimum-compliance one:

The right-hand column costs the developer little during construction, when the scaffolding is up and the walls are open, and costs you a great deal afterwards. That asymmetry is the strongest argument for asking these questions before exchange rather than after.

The one genuine advantage of a developer’s system

Worth saying plainly, because this page is otherwise a list of what to check.

Installation during construction is the cheapest solar there is. No scaffolding hire, no retrofit roof penetrations, no separate visit, no separate margin. The array on a compliant new build is a real asset that a retrofit buyer would pay several thousand pounds for, and it comes with the house.

The critique here is not that new build solar is bad. It is that it was specified for a regulation rather than for you — so the marginal improvements are cheap now and expensive later, and the questions above are how you find out which ones are still on the table.