The paper summarizes the results of systematic and comprehensive analysis to investigate the impacts of inadequate sizing air-conditioning (AC) systems on the overall energy consumption of medium and large office buildings located in representative US cities. The effects of proper sizing on the overall and disaggregated AC systems are evaluated in terms of energy consumption, peak demand, equipment run-time, and indoor thermal comfort. The presented analysis covers the performance of a wide range of AC equipment components that serve US office buildings including packaged rooftop units as well as central cooling plants. The analysis results indicate that oversizing penalties can be significant on the annual energy consumption and electrical peak demand as well as capital costs for both medium and large office buildings. In particular, the reliance on simplified calculation methods and rules-of-thumb to determine equipment capacities can lead to significantly oversizing AC systems for office buildings in the vast majority of US climates. The analysis results indicate that 50% oversizing of AC systems can result in increases in annual energy consumption of up to 91% for large offices and 39% for medium office. Moreover, oversizing increases capital costs required for air-conditioning office buildings and extends cycling periods and associated structural stresses and failures of AC equipment.