Rapidly decarbonizing the power sector is fundamental to confronting the climate crisis. Since fossil fuels still account for roughly three-fifths of utility-scale electricity generation in the United States, ramping up energy savings is a critical component of any plan to achieve a carbon-free grid. This policy brief explains why energy efficiency is important for a carbon-free grid, describes why an Energy Efficiency Resource Standard (EERS) or similar performance standard for efficiency programs is essential for a low-cost, equitable clean energy transition, and summarizes what we can learn from the many states that already have such a policy. It describes the options we have to embed an EERS in a federal Energy Efficiency and Clean Electricity Standard (EECES), with separate savings and supply targets (or how to set a joint target) to achieve clean electricity goals.