Over the last few decades, 30 states and Washington, D.C., have enacted renewable portfolio standard (RPS) programs. These programs vary substantially, with most states having a restriction or preference with respect to whether renewables are located in-state or in-region. This Article takes a cross-disciplinary approach to analyzing these programs: first, by looking at how geographic limitations may run afoul of the dormant Commerce Clause (DCC); and second, by considering empirical research on how geographic provisions affect RPS programs’ cost-effectiveness. Prohibiting or restricting out-of-state renewables from counting toward RPS requirements is likely unconstitutional, but policies applying on a regional basis or pursuing a practical aim are likely to be viewed more favorably by courts. The DCC thus plays an important role in improving uniformity among state RPS programs and promoting free trade and efficiency.