Xcel document describes Version 1 of the the Buildings GHG Mitigation Estimator tool. This tool assists federal agencies in estimating the greenhouse gas mitigation reduction from implementing energy efficiency measures across a portfolio of buildings. It is designed to be applied to groups of office buildings, for example, at a program level (regional or site) that can be summarized at the agency level. While the default savings and cost estimates apply to office buildings, users can define their own efficiency measures, costs, and savings estimates for inclusion in the portfolio assessment. More information on user-defined measures can be found in Step 2 of the buildings emission reduction guidance. The output of this tool is a prioritized set of activities that can help the agency to achieve its greenhouse gas reduction targets most cost-effectively.