Before and since the election, Ed Miliband has spoken of the need to create a vast energy storage network connected to the national grid — to ensure that, in a renewable energy system, the lights don’t go out when the sun is not shining and there is no wind. Part of this, he has said, will be supplied by arrays of giant batteries. He mentioned them again in his conference speech last week, when he called for the “armoury of clean power”.

Enter Field, a battery power storage firm set up in 2021 by Amit Gudka, one of the co-founders of collapsed energy firm Bulb. Field, with five sites already finished or being built, claims to be ready to meet Miliband’s challenge. And Giant Ventures seems to agree: according to data gathered by finance research experts PitchBook, it invested in Field at its outset in 2021.

Now, it is possible that Ed Miliband was unaware that Field was partly financed by Giant Ventures, although this had already been reported. It is also possible that didn’t know his brother was a member of the Giant Ventures advisory board, though that information was also public.

But that would be puzzling. According to Whitehall’s Ministerial Code, newly appointed ministers must formally declare “all interests that might be thought to give rise to a conflict” with their government post: not only their own, but those of their spouse and “close family members”. They must not only avoid any conflict, but also the “perception” that one might arise. It should have been obvious to Ed Miliband that he had a duty to check his brother’s interests.

This year’s declaration is yet to be published, but both Labour and Conservative former ministers told me that declarations of ministerial interests are usually made immediately after taking office, so that civil servants are aware from the start where conflicts might arise. I asked both the energy department and the Cabinet Office, which is responsible for the list, whether Ed had declared David’s role at Giant Ventures, but both refused to comment. Asked when the next list would be published, all they would say is that this would happen “in due course”.

Similarly reticent, Giant Ventures refuses to disclose how much it pays David Miliband, but it seems certain he does receive a stipend: one of his fellow advisory board members, Lord John Browne, the former chairman of BP, states in the Lords register of interests that the position constitutes “remunerated employment”. For Miliband in particular, this “employment” takes a number of forms: not only does he attend board meetings — he also appears at events and in the firm’s videos and podcasts.

As for the insight he provides to the fund, a spokesperson told me that, “like all venture capital firms that are not listed on public markets, we do not publish our investor reports to the general public”. However, PitchBook has identified two firms that look likely to profit from Ed Miliband’s “clean energy superpower” revolution. As well as Field, for instance, there is Beams, which helps homeowners make their properties more energy efficient — a service that should prosper following Miliband’s pledge to impose tough, green standards on both private and public rental homes.

Miliband has also made a specific ministerial decision that will benefit Field. Less than three weeks after the election, Field became the beneficiary of a technical change to the rules governing Britain’s electricity supply system that Miliband personally signed.

The document in question, the Capacity Market (Amendment) Rules 2024, is not an easy read. It relates to the curious way that the grid pays for energy storage: under the “capacity mechanism”, the Government concludes expensive contracts with companies such as Field, who in return guarantee that they will supply a stipulated amount of electricity at a moment’s notice for a given period of time. (In the case of batteries, the limitations of battery technology mean this will be no more than a few hours.) Under this system, the companies get paid whether or not they actually supply any energy.

The cost, which has been rising steadily, is met by adding a levy to customers’ electricity bills. Energy supply expert Gordon Hughes, emeritus professor of political economy at Edinburgh University, told me that by the beginning 2026, the annual cost is likely to be more than £2 billion, the equivalent of £70 per household, and by 2030, up to £5 billion.

Until Ed Miliband changed the rules, the problem for companies like Field was that when batteries are used and recharged repeatedly, their capacity deteriorates. This meant that battery firms had to pay to install more capacity than their contracts said they needed when their batteries were new, so that they would not be running the risk of breaching their contracts when their capacity declined. This was because the old rules said they were not allowed to add more capacity if and when it was required. “This change is clearly advantageous to them,” says Hughes, “because it allows them to reduce their upfront costs. It is also likely to increase their profits.”

This was not the only decision Miliband made on 18 July that may come to benefit Field. On the same day, he signed a letter to the national grid setting out details of the capacity mechanism’s future, which will create lucrative opportunities for firms like Field to build more batteries, enhancing their profits and the value of investments made by venture capital firms.

After the Department for Energy Security and Net Zero told me it could not comment as to whether Ed Miliband had declared his brother’s interest in Giant Ventures, I sent an email setting out details of how a conflict, or at least the appearance of one, might occur. But Its spokesperson refused to address this, saying only: “There is an established process in place for the declaration and management of interests held by ministers. This ensures that steps are taken to avoid or mitigate any potential or perceived conflicts of interest. The List of Ministers’ Interests will be published as part of this in due course.”

Giant Ventures said it did not wish to comment, while David Miliband failed to respond at all.

This is unsurprising. Since leaving Parliament, David Miliband has been reluctant to speak publicly about his relationship with Ed. But on one of the few occasions that he has, in 2015, he told The Guardian: “I’ve always said you remain brothers for life.”