Using data on hourly outputs and spot prices for a sample 25 wind and nine solar generating plants covering all seven U.S. ISOs for 2011 and up to 12 adjacent months, this study examines capacity factors, average output values, and several aspects of intermittency. Most performance measures studied vary substantially within and between ISOs, and some vary substantially over time. Implications for research, market design, and policies to support renewable generation are briefly discussed.