The massive roof of the Vatican's "Nervi Hall," where popes hold general audiences and concerts are performed, has been covered with 2,400 photovoltaic panels Â— but they will not be visible from below, leaving the Vatican skyline unchanged.
The new system on the 5,000 square meter roof will provide for all the year-round energy needs of the hall and several surrounding buildings, producing 300 kilowatt hours (MWh) of clean energy a year.
The system, devised by the German company SolarWorld, will allow the 108-acre city-state to cut its carbon dioxide emissions by about 225,000 kilograms (225 tonnes) and save the equivalent of 80 tonnes of oil each year.
The Holy See's newspaper said that the Vatican planned to install enough renewable energy sources to provide 20 percent of its needs by 2020, broadly in line with a proposal by the European Union.
The 1971 Nervi Hall is named after the renowned architect who designed it, Pier Paolo Nervi, and is one of the most modern buildings in the Vatican, where most structures are several centuries old. The hall can hold up to 10,000 people.
It has a sweeping, wavy roof which made the project feasible and the solar panels virtually invisible from the ground. Church officials have said the Vatican's famous skyline, particularly St Peter's Basilica, would remain untouched.
An editorial in a recent newspaper appealed for greater use of renewable energy.
"The gradual exhaustion of the ozone layer and the greenhouse effect have reached critical dimensions," the newspaper said.
By producing its own energy the Vatican will become more autonomous from Italy, from where it currently buys all its energy. The Vatican is surrounded by Rome.
Pope Benedict and his predecessor John Paul put the Vatican firmly on an environmentalist footing.
Benedict has made numerous appeals for the protection of the environment. The Vatican has hosted a scientific conference to discuss the ramifications of global warming and climate change, widely blamed on human use of fossil fuels.
Environmentalists praised the pope last year after he made a speech saying the human race must listen to "the voice of the earth" or risk destroying the planet.