The photo-voltaic panels placed on the roof of Dunster Castle which was owned by the Luttrell family for some 600 years before being given to the National Trust in 1976.
A number of the National Trust's properties have already had micro-generation renewables such as solar panels installed, but Dunster is the first Grade I - the most protected - to have the technology.
Because of the listed building constraints, the panels are sited on a specially-designed steel frame which makes them easily removable and they are not visible from the ground.
The installation is part of a three-year carbon reduction project at the castle. It is calculated that the 24 panels on the eastern roof will generate 5,500 kilowatt-hours a year, saving around 3,000kg of carbon dioxide annually and paying back the carbon footprint of the frame and technology in around four years.
The £55,000 scheme was funded by Barclays, the Government's Low Carbon Buildings Programme and the National Trust.
Property administrator Stephen Hayes said the move was more about raising awareness.
Mr Hayes said: "The National Trust philosophy at the moment is to drive towards looking at the environment and improving our environmental performance as an organisation and inspire others to do the same."