The Property Services Grant-In-Aid is the annual funding provided by the Department for Culture, Media and Sport to The Queen’s Household to meet the cost of the maintenance of the Occupied Royal Palaces.
The Prince of Wales’s principal office, where most of his staff work, is in Clarence House. The cost of maintaining the fabric, plus the cost of the utilities and fixed-line telephones (but not other costs for the London office), is met from the Property Services Grant-In-Aid.
Clarence House is the London office and official residence for The Prince of Wales, The Duchess of Cornwall, Prince William and Prince Harry. It is used for official dinners, receptions and meetings, plus being the offices for official staff.
There are also offices for official staff at Highgrove and Birkhall (in Scotland), The Prince of Wales’s private residences. Therefore a portion of their costs are allocated to "official duties"
Duchy of Cornwall Income and Expenditure