Lawrence Berkeley Laboratory is developing a PC-based computer tool, the Retrofit Energy Savings Estimation Model (RESEM) which can provide high-quality estimates of energy savings, based on actual pre-and post-retrofit utility bills. Designed to be used by state and regional energy office staff who have little energy modeling expertise and access to only limited information regarding a building and its retrofits, RESEM hides much of its sophistication behind a simple, intuitive interface. Descriptions of the development of RESEM and of its characteristics provide an overview of an interesting and instructive exercise in the careful packaging of an ASHRAE-based energy estimation model into a robust, generalized tool that accomplishes a complete retrofit savings analysis of a building without demanding extensive engineering expertise on the part of the user. We describe software design goals and approaches developed for: providing a rational interface and maximal self-containment; development of building prototypes and parameter defaults based on minimal information from the user; a concise and efficient way of describing retrofits; the calibration of the building description and its simulated performance against utility bills; and the allocation of energy savings among multiple retrofits, together with cost-effectiveness estimates. Future needs and capabilities to respond to them are also discussed.