The major policy instruments for mitigating climate change actually in use in the U.S. are subsidies provided to renewable energy. A popular means of subsidy is through state-level Renewable Portfolio Standards (RPSs), requirements enacted by many states that require a certain fraction of electricity must be derived from renewables. However, many state RPSs are infeasible because of “cost cap” provisions that do not permit renewables to be sufficiently competitive, and feasibility is generally not assessed before legislation is passed. The RPS calculator allows the user to explore the conditions for RPS success or failure in different states. The user can analyze and modify existing state statutes or design new statutes in states that do not have them. Users can explore the effects of parameters such as electricity prices, generation costs for wind and solar, interest rates, technology carveouts, and cost cap structure. Features under development include extension to all U.S. states (currently only IL and CA) and spatial variation in wind speed (wind capacity factor map).