Market Watch  ·  Issue 11  ·  Tuesday 6 October 2026

Watts Happening

 

The UK energy newsletter that doesn't work for the industry

The Spark

On an existing home, a 4 to 10 kW solar system cost £1,651 per kW on average to fit in the year to March. On a new build, it cost £1,440.

New homes get solar cheaper per kW than existing ones. They also get less of it. Of the 24,902 new-build home systems in the government's figures, 86% were up to 4 kW. On existing homes, it was 40%.

Existing homes are moving to bigger systems. New homes are being fitted to a regulation.

This Week in Watts

Why new-build homes get five solar panels on a roof built for eight

Walk past any new estate and you will see it. Two panels on one side of a roof. Five in an odd shape where eight would fit. I see it on site all the time, and the reason is in the building regulations.

Under England's 2021 rules, which still apply to homes being built today, a new house is tested against an imaginary version of itself: a gas boiler, radiators and a set amount of solar. The real house has to match that benchmark's energy and carbon figures. The regulations say it can get there by "any combination" of insulation, efficient heating and low-carbon technology. Nothing requires a single panel.

So solar becomes the top-up. Insulation and heating close most of the gap, and the developer fits as many panels as the sum needs and no more. The government's own example of a heat pump home that should pass the test has no solar at all.

New build vs existing home

Domestic solar, year to March 2026. Source: DESNZ solar PV cost data (MCS installs, GB)

  New build Existing
Systems up to 4 kW 86% 40%
Average cost per kW, up to 4 kW £1,518 £2,109
Average cost per kW, 4 to 10 kW £1,440 £1,651

Then the roof itself. Many new builds use in-roof panels, set into the roof in place of tiles. They look better than panels bolted on top. They also turn adding panels later into a roofing job. The minimum fitted on day one tends to be the system for life.

The rules change on 24 March 2027. From then, most new homes in England must generate electricity on site, sized to match panels covering 40% of the ground floor. For a house with a 50 m² ground floor, that is about 4.4 kW. The old benchmark worked out at about 3.1 kW, and was never compulsory.

The catch is the transition. Where plans went in before 24 March 2027, a home can still be built to the old rules, as long as work on it starts before 24 March 2028. New homes sized to the 2021 calculation will be sold for years yet.

The system on a new roof was sized to pass a calculation. The bill it was meant to cut belongs to whoever moves in.

The Bill Buster

Buying off-plan? Ask about the solar before the roof goes on

If you are reserving a new-build home, the solar has already been sized, but on an unbuilt plot it may not be fitted yet. That is the cheapest point to go bigger.

Ask the developer three things, in writing. What size the system is, in kW, not how many panels. Whether it is in-roof or on-roof, because that decides whether you can add to it later. And what it would cost to fit a larger system before the roof is finished.

Some will say no. The answer still tells you whether the system was designed for the house or for the paperwork.

→ Ask: what kW is specified, and what would bigger cost before the roof goes on?

Watts on the Market

Unsold this week  ·  Reader resource

A new home's solar is sized to pass an energy calculation. Your own energy costs are a different number, and few people have seen it. The Cost of Inaction calculator runs your current setup forward 25 years, on your own bill and property type, across two inflation scenarios. No installer at the end of it.

→ Run your 25-year number

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