The new system will heat the hall where Pope Benedict XVI holds his weekly general audience.
The hall was built in 1971, making it one of the Vatican's more modern structures. In order to power it, the roof was covered with 2,400 photovoltaic panels.
Officials said the panels are nearly invisible from the ground and do not degrade the beauty of the architecture. The panels are expected to generate about 300 kilowatt-hours per year.
The panels will power not only the meeting hall, but several nearby buildings as well. They were built and installed by a German company called SolarWorld.
By replacing traditional power with renewable energy, the Vatican can cut its carbon dioxide emissions by about 225,000 kilograms per year.
The Vatican's goal is to install enough renewable energy sources to provide 20 percent of its needs by 2020.