Design engineers commonly oversize HVAC systems with the justification of needing a reasonable safety factor to manage periods more extreme than the specific design conditions. Unfortunately, the safety factor easily becomes excessive. The design engineers minimize their professional risk, and by doing so they are actually asking the building owner to pay an immediate penalty due to increased first cost of equipment and an ongoing penalty due to maintenance and energy use implications. The penalties associated with excessive safety factors are often not communicated to the client. This paper presents the results of a study of "rightsizing" rooftop HVAC systems. The study included intensive interviews with HVAC designers investigating the design process and extensive field measurement of rooftop units (RTUs) during peak cooling conditions. This paper focuses on defining the signature of oversizing, i.e. how to use the physical measurements to quantify the degree of oversizing of an RTU and how to estimate the penalty of oversizing in terms of energy consumption and peak electricity demand. Utility companies incentive programs have not yet identified mechanisms for incentivizing rightsizing of HVAC system. The methodology described in this paper can be used as the basis for such programs. (C) 2010 Elsevier B.V. All rights reserved.