"This major investment in UK renewables is a vote of confidence for building in Britain," declared the prime minister.

Andy Burnham is obviously correct that the two Vanguard windfarms off the coast of Norfolk are huge investments. Together, they require almost £13bn of debt financing. But he can't claim they represent a fresh expression of confidence. The project has been on the runway since the German utility RWE, the developer, was awarded a contract to build the windfarms in January's renewables auction.

Instead, the new news on Wednesday was how the project will be financed, which is where the story becomes interesting. On the equity front, RWE said it had sold a 50% share in the Vanguard schemes to KKR, the giant US private equity and infrastructure investor. On the debt side, £12.8bn has come from "a consortium of 35 international banks", including £1bn from the UK's own national wealth fund (NWF).

It is fair enough, one might say, for the NWF to get involved – directing capital to energy transition projects is, in part, what it was set up to do. But here's another key aspect of its role: the state-backed body is not meant to be a substitute for private capital; instead, its funding is supposed to be "additional", in the terminology, to ensure infrastructure is built that might not otherwise be.

Here is how the NWF describes the process: "For each investment we are considering, we test and assess the extent to which impact would not have materialised without us, so we are confident we are delivering financial and non-financial value that would not otherwise be achieved."

So how does that work with the Vanguard windfarms? Would they have been scrapped if the NWF had not chipped in its £1bn? That seems highly unlikely.

Consider that RWE is one of Europe's biggest electricity generators and that KKR is a giant among private equity houses. These rich firms are experienced at raising capital. Consider, too, that the main way in which offshore wind projects are de-risked for developers is the auction process that sets a guaranteed inflation-linked price for their electricity for 20 years. In this case, the price is £91.20 a megawatt hour (in 2024 money), or high by historic standards.

If the cost of borrowing has risen post-auction – and it has – that's unfortunate for RWE and KKR. They might have to pay a bit more for their debt, or pony up more equity, both of which would depress their returns. But that is an everyday risk of commercial life. Why did the NWF, having stayed away from fixed offshore wind until now, think it had to join a lending syndicate on a project where it is a stretch to think its contribution was the make-or-break element?

The NWF's argument is that its involvement "widens the pool of capital available to offshore wind developers and helps to meet unprecedented liquidity requirements after the success of AR7 [the last auction round]". In other words, the sector will need so much capital over coming years that the state-backed lender needs to be on standby.

It's a defence, but it will sound very appealing from the point of view of developers. They can bid for an index-linked contract that guarantees revenues for 20 years; then, if borrowing conditions are tight, they are invited to think that the state will lend a hand to try to ease funding pressures. No wonder KKR wanted in. The NWF, note, says it is lending at market rates. Even so, its definition of how it adds "additional" value seems to have become very stretched.

Many of the NWF's other big loans, such as billions for Sizewell C and £599m for Rolls-Royce's development of small modular reactors, are far easier to understand. Those projects genuinely might not have happened without state backing. The same applies to the myriad of its smaller investments in battery technology and so forth.

Offshore wind generation, by contrast, is an established market with an effective mechanism – the contract-for-difference – for incentivising developers. The NWF's pot of capital is not bottomless. Better targets for loans right now might be anything to do with electrification, meaning the take-up of heat pumps and electric vehicles, or storage solutions. As half the energy world is pointing out, it is where the big decarbonisation gains are available.

A bit of state activism to accelerate energy transition is a good thing, many of us would say. But, as argued here previously, more scrutiny of that "additionality" principle is overdue. There is little point replicating what the private sector would do anyway. When a huge private equity firm, KKR, is getting a helping hand because funding conditions are a bit rough, it is fair to ask if the state is achieving the best bang for its finite buck.