Charles installed solar panels on part of the Windsor Castle estate known as the Lord Chamberlain's Upper Store, replacing the existing lead roof, and the plan will be to add more panels in the future, officials said.

The royal family has Bentley limousine cars and these will be converted to run on biofuel in the next year as an interim step before the household's fleet of cars eventually becomes electric, the officials said.

The king has long been outspoken on conservation and in 2020 described global warming and climate change as the greatest threat that humanity has faced.

Palace officials provided the information as they explained details of the annual report into the king's taxpayer-funded spending and income, known as the Sovereign Grant, published on Wednesday.

This showed that official expenditure for 2023-24 had been 89.1 million pounds ($115 million), above the 86.3 million pounds allocated, forcing the royal household to dip into reserves to fund the deficit, though by less than last year.

Overall expenditure fell by 17% compared with the financial year 2022-2023, helped by a 10.8 million pound fall in the cost of property maintenance.

Additional income to supplement the Sovereign Grant more than doubled to 19.8 million pounds helped by a rebound in ticket sales to visit royal palaces, which almost returned to the levels before the COVID-19 pandemic.

The royal family also plan to take delivery of two new helicopters this financial year, the accounts show. The report justified the cost of the new helicopters, saying the existing ones are 15 years old, and it will help members of the royal family complete multiple engagements on a single day.

Overall, travel costs amounted to 4.2 million pounds, with the biggest overseas expense the king and queen's state visit to Kenya which cost 167,000 pounds.

($1 = 0.7745 pounds)