DrivePak is a variable-frequency-drive (VFD) upgrade that increases the energy efficiency of packaged rooftop HVAC equipment. DrivePak works by changing the operation of HVAC fans from constant speed to variable speed by matching fan speed to actual load requirements. DrivePak users report an average of 15 percent overall facility energy savings. Simple paybacks commonly are in the 1.5-to-2.5-year range; with larger units, they can be less than a year.