The House on Friday began debate on a massive bill designed to cap U.S. greenhouse-gas emissions. Among the bill's provisions: -- Emissions from a large sector of the U.S. economy, including power plants, factories and auto tailpipes, will be required to be cut 17 percent below their 2005 levels by 2020, and 83 percent below those levels by 2050. -- These reductions would be managed by requiring emitters to amass buyable, sellable 'credits' equal to their pollution. -- About 85 percent of these credits would be given away, many of them with the mandate that electricity distributors sell them and use the proceeds to soften the blow of rising energy prices. Environmentalists had wanted the government to auction them all off. -- Electricity producers would be required to get at least 15 percent of their energy from renewable sources by 2020, with as much as 5 percent more energy saved from new efficiency measures. Two figures must add up to 20 percent. -- Polluters could also balance out some of their emissions by purchasing carbon 'offsets,' which are official certificates that greenhouse-gas emissions have been avoided or taken out of the air. In a last-minute amendment, oversight over offsets generated on farms was taken from the Environmental Protection Agency and given to the Department of Agriculture. -- A new Clean Energy Deployment Administration funded with $7.5 billion in 'green bonds' would provide government money to private companies investing in environment-friendly technologies. 