Buildings are required to follow standard operational procedures during emergency evacuation. In addition to people evacuating the building, one of the recommended steps during a fire evacuation is to shut down the air handling units (AHUs) of the heating, ventilation, and air conditioning (HVAC) system to prevent smoke from spreading in the building via the air ducts. Shutting down the AHU will inevitably cut-off cooling, resulting in internal temperatures rising steeply particularly on hot days. This phenomenon imposes considerable power demand on the HVAC to rapidly cool the building down during reoccupation. In this article, we study the energy implications of post-evacuation scenarios. Our contributions are threefold: 1) we quantify power excursion caused in 43 evacuation events across 14 buildings of a university campus using a data-driven building thermal model. We show evacuations during summer season can result in power consumption up to 150% above the power demand threshold; 2) we develop a method to reschedule planned evacuations in order to eliminate the power excursions while adhering to building evacuation standards; and 3) we develop a formal optimization framework to minimize the energy costs during planned and emergency evacuations without compromising the desired thermal comfort temperatures by intelligently cooling the building post evacuation. This is the first study to understand and reduce the HVAC power consumption associated with building evacuation events.