When calculating annual energy use, it is necessary to make assumptions about how the proposed building is operated. Operating assumptions include thermostat settings, number of occupants, receptacle loads, process loads, hot water loads, and operation schedules for heating, ventilation, and air-conditioning (HVAC) systems, lighting systems, and other systems. Sometimes these data are known with some certainty, and other times (for instance, for speculative buildings), it is necessary to make estimates. Some of these inputs are prescribed (fixed for the proposed and standard design buildings and cannot be changed), while others are defaults.