Policymakers at the local, state, and federal levels are increasingly turning to building performance standards (BPSs) to reduce buildings’ contributions to climate change. A key question in designing BPSs is what “metric” the standards should use to gauge a building’s performance. This Comment provides general background information on the case for regulating energy use in buildings, reviews the two general categories of metrics in existing BPSs and explains why an energy efficiency-based standard is superior to a greenhouse gas-based standard, and highlights the findings from a study of New York City’s landmark BPS, Local Law 97 of 2019, that underscore the disadvantages of regulating greenhouse gas emissions as opposed to energy use in a BPS.