An on-site controller for an HVAC system is operated to maintain temperature and humidity set points inside an enclosed facility. A weather station inside the facility records microclimate data pertinent to environmental conditions inside the facility, and a weather station outside the facility records microclimate data pertinent to meteorological conditions outside the facility. A summing network compares the inside microclimate data with the outside meteorological data to establish an actuating signal. A throttle, responsive to the actuating signal, varies the power demand for the HVAC system to maintain the predetermined set points for environmental conditions inside the facility.