This article investigates the effect of air-conditioning (AC) systems on air temperature and examines their electricity consumption for a semiarid urban environment. We simulate a 10-day extreme heat period over the Phoenix metropolitan area (US) with the Weather Research and Forecasting (WRF) model coupled to a multilayer building energy scheme. The performance of the modeling system is evaluated against ten Arizona Meteorological Network (AZMET) weather stations and one weather station maintained by the National Weather Service (NWS) for air temperature, wind speed, and wind direction. We show that explicit representation of waste heat from air-conditioning systems improved the 2 m-air temperature correspondence to observations. Waste heat release from AC systems was maximum during the day but the mean effect was negligible near the surface. However, during the night, heat emitted from AC systems increased the mean 2 m-air temperature by more than 1 °C for some urban locations. The AC systems modified the thermal stratification of the urban boundary layer promoting vertical mixing during nighttime hours. The anthropogenic processes examined here (i.e., explicit representation of urban energy consumption processes due to AC systems) require incorporation in future meteorological and climate investigations to improve weather and climate predictability. Our results demonstrate that releasing waste heat into the ambient environment exacerbates the nocturnal urban heat island and increases cooling demands.