Energy Efficiency Guide for Existing Commercial Buildings: The Business Case for Building Owners and Managers is part of the Advanced Energy Design Guide series and is designed to address existing buildings rather than new construction. Buildings that are energy-efficient, green, and sustainable can have significant long-term value. This represents a tremendous opportunity for building owners and real-estate developers, with great potential for a positive return on investment.This Guide is the first installment in what is expected to be a three-part series and is aimed at providing the business case for energy efficiency to building owners and managers; it provides the rational for making economic decisions related to improving and sustaining energy efficiency in existing buildings. The goal of this publication is to enable building owners to evaluate current operations and conduct the economic analysis necessary to make wise decisions regarding improvements. The Guide demonstrates how to benchmark performance against comparable buildings, illustrates how energy use and cost can be reduced by at least 30%, and describes how to do all of this while minimizing capital investment and maximizing return on investment. In addition, the Guide identifies additional measures that can achieve even greater energy efficiencies and cost savings in economical ways.