After voluntary trials in the UK and Denmark, Phase I began operation in January 2005 with all 15 member states of the European Union participating. The program caps the amount of carbon dioxide that can be emitted from large installations with a net heat supply in excess of 20 MW, such as power plants and carbon intensive factories, and covers almost half (46%) of the EU's Carbon Dioxide emissions. Phase I permits participants to trade among themselves and in validated credits from the developing world through Kyoto's Clean Development Mechanism. Credits are gained by investing in clean technologies and low-carbon solutions, and by certain types of emission-saving projects around the world to cover a proportion of their emissions.