With increasing code expectations on lighting, mechanical and electrical systems more pressure is placed on the architectural design of buildings. However, little is known about the geometric sensitivity of the architectural form related to energy performance. Various studies highlight the reciprocal relationship between certain building types and specific climates. The studies are limited in that they explore single building use types, single climates, and simple building geometry. This paper will describe a custom tool titled Rapid Energy Performance Simulator (REPS) allowing design teams to explore in any climate the energy performance of extruded footprints, building orientation, window wall ratio, and aids in prioritizing shading. The REPS tool is designed to work with few initial inputs, novice software users, and early conceptual design. Analysis of resulting geometric results across climate zones provides insight into the geometric sensitivity of the building’s shape on energy efficiency and validates the value of REPS for early building performance simulation of energy use.