Apparently the National Trust property Dunster Casle has installed 24 solar panels. It becomes the first NT Grade 1 listed property to introduce this sort of renewable enrergy technology.
It will produce the equivalent of the energy consumed by two ordinary family homes. The cells will generate 5,500 kilowatt-hours (kWh) per year of electricity, meaning that the Castle will not to "import" that electricity from the fossil-fuelled National Grid and save almost 3000 kg of CO2 a year. That's the equivalent of travelling almost 14,000 miles in an average car or about 280 journeys by car from the nearest town (Taunton) to visit the Castle each year.
The design has taken account of aesthetics and the historic importance of the building. There is no loading or direct contact of the panels, or frame, on the roof itself and the panels will not be visible from ground level. The design is fully reversible, should new technologies develop in the future.
The panels will take about 4 years to "pay back" the CO2 cost in producing them.