Use of marginal energy prices, instead of average energy prices, represents a theoretically valuable and challenging refinement to the usual life-cycle cost analysis conducted for proposed appliance energy efficiency standards. LBNL developed a method to estimate marginal residential energy prices using a regression analysis based on a nationally representative sample of actual consumer energy bills. Based on the 1997 Residential Energy Consumption Survey (RECS), national mean marginal electricity prices were estimated to be 2.5% less than average electricity prices in the summer and 10.0% less than average prices in the non-summer months. For natural gas, marginal prices were 4.4% less than average prices in the winter and 15.3% less than average prices in the non-winter months.