Solar panels that could generate up to 20% of the electricity needed at the Grade I listed Dunster Castle in Somerset have been switched on.
The panels are not visible from the ground
A number of National Trust properties use renewable energy but the trust said it was the first Grade I listed building to be given the technology.
The panels are easily removable and are not visible from the ground.
The £55,000 scheme will not pay for itself financially but is expected to save about £550 per annum.
A National Trust spokesman said the panels at Dunster Castle demonstrated how renewable energy could be harvested even on conservation sites without affecting their character.
"We want all of our properties, wherever appropriate, to examine the option of using renewable energy sources and we hope to help other people and businesses to do the same," he added.
Dunster Castle was owned by the Luttrell family for some 600 years before being given to the National Trust in 1976.