Solar panels are being installed on Paul VI auditorium's roof in the Vatican.
A group of engineers started last week placing 2,700 solar panels on the roof in place of the deteriorating cement roof tiles.
The photovoltaic cells will convert sunlight into electricity, producing enough power to light, heat or cool the 6,000 seat hall, engineers say.
The panels, reportedly worth £750,000, were donated as a gift to the Pope by a Bonn-based company, Solar World.
Since Pope Benedict was elected in 2005, he has made efforts to conserve global resources and openly criticized the "unbalanced use of energy" in the world.
An engineer working on the site said: "With this plan, in two weeks we avoid 200 tons of carbon dioxide, the equivalent to 70 tonnes of oil."
When the hall is not being used, the surplus energy produced will be fed into the Vatican's power network.