We analyze policies to promote renewable sources of electricity. A renewable portfolio standard raises electricity prices and primarily reduces gas-fired generation. A "knee" of the cost curve exists between 15% and 20% goals for 2020 in our central case, and higher natural gas prices lower the cost of greater reliance on renewables. A renewable energy production tax credit lowers electricity price at the expense of taxpayers and thus limits its effectiveness in reducing carbon emissions; it also is less cost-effective at increasing renewables than a portfolio standard. Neither policy is as cost-effective as a cap-and-trade policy for achieving carbon emissions reductions.