A pair of gas-guzzling Bentleys are not the most obvious candidates to burnish the monarch’s green credentials. But news that King Charles is converting his chauffeur-driven luxury vehicles to run on biofuels was this week billed as a small step in a bigger plan to reduce emissions – perhaps the equivalent of lesser mortals separating paper from plastic in the weekly rubbish.

“The two existing state Bentleys will undergo refurbishment in the coming year to enable them to run on biofuel,” said Sir Michael Stevens, the keeper of the privy purse, adding that it was an interim measure in advance of “the next generation of state vehicles being fully electrified” and part of a “wider plan to make a significant impact on our carbon emissions in the years ahead”.

The announcement was swiftly followed by news of a deal between the crown estate, the monarchy’s property arm, and Great British Energy, Labour’s new publicly owned energy company, to build offshore windfarms to power 20m homes. The crown estate, which will receive a healthy share of any profits, owns most of the seabed up to 12 nautical miles from the coastline.

Both measures reinforced the image of Charles as the “green king”. Tony Juniper, the chair of Natural England and former executive director of Friends of the Earth, who has co-authored two books with Charles, said: “There is no doubt that our monarch, our head of state, is a very committed and very effective environmentalist.”

Among other changes detailed in this week’s sovereign grant annual report were the first solar panels to be fitted at Windsor Castle, with heat pumps to follow; and gas lanterns at Buckingham Palace being given new electrical fittings to “improve their energy efficiency while preserving their historic look and glow”.

Stevens said the changes were “driven by a determination to place sustainability at the heart of our operations and inspired by his majesty’s leadership in this sphere”.

The king’s devotion to environmental causes has not inhibited the royal family from taking delivery in the coming months of two new helicopters to replace the existing 15-year-old models.

“The use of helicopters is a key component of delivering engagements by his majesty, as head of nation,” the report said. However, the new helicopters will be able to operate with 50% sustainable aviation fuel (SAF), the maximum industry standard.

Last year, Charles appointed a new head of sustainability to accelerate his net zero ambitions. “It is ever clearer that urgent action is needed for a zero-carbon, nature-positive future,” the report said.

“The royal household recognises the twin global challenges of climate change and biodiversity loss and its role in taking action, demonstrating leadership, and encouraging and inspiring others.”

The royal electric bill rose from £1.3m in 2023 to an estimated £2.2m this year, although this was attributed to a change in tariff rather than an increase in consumption. The royal gas bill went down from £1.4m to £1.2m. Waste produced across the royal household fell 12%.

Emissions from royal business travel were 17% higher than the previous year, mainly because in 2022 there was a drop due to Queen Elizabeth’s death and King Charles’s accession. “Travel emissions will continue to fluctuate each year depending on requests from government for tours involving long-haul charter flights,” the report said.

Charles has espoused environmental causes for more than half a century. He made his first speech on green issues at the age of 21, warning of the threats from plastic waste and chemicals dumped into rivers and seas, and air pollution from industry, vehicles and planes.

At the time, his views were seen as eccentric. When he suggested recycling bottles at Buckingham Palace or argued for organic farming, he was considered “completely potty”, he acknowledged in 2020.

Juniper said: “He had to put up with some really quite unpleasant ridicule during the early years of raising these questions. But he knew he was in a bigger battle than just dealing with a few sceptics, that this was an existential threat.

“And so he stuck with it. He was a real pioneer for many years when mainstream figures wouldn’t acknowledge the scale of the threat, and had to put up with a lot of pushback from different people. And in the end, of course, he was ahead of the consensus.”

Now he is king, he has less freedom to speak out publicly on the issues close to his heart. But there is nothing to stop him making changes to the way the monarch’s castles, palaces and other homes are run.

In April, the Sandringham estate applied for planning permission to site 2,000 solar panels on its approximately 8,000 hectares (20,000 acres), part of an “ongoing commitment to sustainability and promoting environmental practices”.

The system was designed “to meet the majority of the estates’ electricity demand, with a modest amount of spare capacity exported to the grid”, its application said.

Since Charles took over the running of the estate in 2017, its farms have moved to organic agriculture; trees and hedgerows have been planted, and wildlife has been encouraged.

Juniper said: “He’s trying to be consistent in his personal life with a far bigger contribution that he’s made, which is brokering agreements across chasms of disagreement that have dogged progress over many decades.

“His interests have spanned agriculture, the food system, pollution, waste, biodiversity, rainforests, security and global heating. And he’s engaged with all of it from end to end.

“His proud history of advocacy and change-making leaves him in a very strong position as monarch to embody those values.”