The King's two state Bentleys will be converted to run on biofuel within the next year, as part of a wider sustainability push.
Longer term, the plan is to switch to a fleet of official electric cars.
Annual accounts for the Royal Household, published on Wednesday, revealed a string of new, more environmentally friendly measures.
Taxpayer funding for the monarchy's running costs was £86.3m in 2023-24, the same as for the previous year.
This funding, which covers such things as official visits and residences, is known as the "Sovereign Grant".
Charles has been campaigning on climate issues for decades but the latest accounts - for the first full financial year of the King's reign - indicate he is determined to do things differently.
Windsor Castle will be fitted with solar panels for the first time while the gas lanterns at Buckingham Palace are being repurposed with specially designed electrical fittings to improve their energy efficiency while keeping their historic look and glow.
Sir Michael Stevens, Keeper of the Privy Purse, said the environmental initiatives were "driven by a determination to place sustainability at the heart of our operations and inspired by His Majesty’s leadership in this sphere".
Charles has previously said that people thought he was "completely dotty" when he started talking about environmental issues in the 1970s.
The annual accounts were delayed for a month because of the General Election.
They revealed the royal household would take delivery of two new helicopters over the coming year, to replace the existing 15-year-old ones.
The report said the helicopters were "a key component" in enabling the King and the royal family to carry out their engagements, allowing them to access remote regions of the UK.
There has also been work to increase the use of sustainable aviation fuel for royal flights where practicable.
Taken as a whole, the report said there had been a decrease of 3% in natural gas and heating emissions across the Royal Estates, although total greenhouse gas emissions did however increase slightly from the previous year.
This was largely due to increased business travel emissions in comparison with 2022-2023, when royal travel was limited by events around the change of reign.
A 10-year £369m project to repair Buckingham Palace is under way.
A separate National Audit Office (NAO) report on Tuesday concluded that the renovation work had been well managed overall but it warned that structural damage and the discovery of asbestos, which had led to cost increases, "could have been foreseen".
It said 82% of operational improvements were now complete.
Finished work included installing new boilers, a switch panel and back-up generators; moving water tanks; and removing dangerous electrical cabling, according to the NAO.
The Duke and Duchess of Sussex's former home, Frogmore Cottage, is still empty more than a year after they officially left the property.
At the royal finances briefing, palace officials revealed there were no new tenants in the Grade II Crown Estate property in Windsor Home Park.
'Swiftly changing circumstances'
The period covered by the accounts - from April 1 2023 to March 31 2024 - covers the celebratory moments during the coronation and crowning of the King and Queen.
But they also span a more difficult time for the family, following the cancer diagnoses of both the King and the Princess of Wales earlier this year.
Sir Michael acknowledged this, saying some of the events being referred to might "already seem like ancient history, given all the royal news that has unfolded since".
"In the early part of 2024 came the sad news that both His Majesty the King and the Princess of Wales would be withdrawing from public-facing duties temporarily, to prioritise their treatment and recovery from cancer.
"This inevitably impacted on the number and nature of engagements that had been planned - though may I say how encouraging it is to see the King back performing so many public duties and, more recently, the princess similarly well enough to join the King's Birthday Parade and the men's Wimbledon final."
Taken as a whole, there were more than 2,300 official engagements by members of the royal family in the UK and overseas, compared with more than 2,700 last year.
Charles undertook 464 official engagements despite his cancer diagnosis.
The most expensive of those was his state visit to Kenya, which cost £167,000.
"Behind the scenes, the work of the Royal Household continued apace, even throughout the latter course of the financial year, with His Majesty still performing his full state duties, Her Majesty taking on a greater share of public engagements, and their support teams adapting swiftly to the changing circumstances," Sir Michael added.
