Pope Benedict XVI is to become the first pontiff to harness solar power to provide energy for the Vatican, engineers say.
The Vatican is considering solar panels on other buildings
The deteriorating cement roof tiles of the Paul VI auditorium will be replaced next year with photovoltaic cells to convert sunlight into electricity.
The cells will generate enough power to light, heat or cool the hall, the Vatican engineers say.
Last year the Pope urged Christians not to squander the world's resources.
The BBC's David Willey in Rome says the Vatican is now setting an example to the rest of Italy, which has been slow in harnessing solar.
Pope Benedict has criticised "the unbalanced use of energy" in the world.
Last year he said environmental damage was making "the lives of poor people on earth especially unbearable".
The Paul VI auditorium was designed by architect Pier Luigi Nervi and built in 1969.
The cement panels on its roof have deteriorated and were due to be replaced anyway, said Vatican engineer Pier Carlo Cuscianna.
When the 6,000-seat hall is not in use, the surplus energy will be fed into the Vatican power network.
The Vatican is considering placing solar panels on other buildings although St Peter's Basilica and other historical landmarks will not be touched.