In addition to cutting greenhouse gas emissions, supporters say it provides a guaranteed market for renewable power, thereby spurring investment in the sector and helping reduce costs as renewable technologies develop economies of scale. They are pushing for a federal law requiring utilities in all states to buy renewable power.
Opponents say a federal law is unfair, as some states (think Arizona for solar or North Dakota for wind) are more blessed with renewable energy sources than others.
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Last updated September 25 2008: 3:43 PM ET