Using a panel data over 50 US states and years 1991-2007, this paper uses a state fixed-effects model with state-specific time-trends to estimate the effects of state policies on the penetration of various emerging renewable electricity sources, including wind, biomass, geothermal, and solar photovoltaic. Renewable portfolio standards with either capacity or sales requirements have a significant impact on the penetration of all types of renewables-however, this impact is variable depending on the type of renewable source: it is negative for combined renewables, wind, and biomass; and positive for geothermal and solar. Further, clean energy funds and required green power options mostly result in increasing the penetration of all types of renewables. On the other hand, voluntary renewable portfolio standards as well as state green power purchasing programs are found to be ineffective in increasing the penetration of any type of renewable source. Finally, economic variables, such as electricity price, natural gas price, and per capita GDP as well as structural variables, such as league of conservation voters rating and the share of coal-generated electricity are found to be generally insignificant, suggesting the crucial role of policy in increasing the penetration of renewables. (C) 2011 Elsevier Ltd. All rights reserved.