A novel process for activating available compressors in multiple compressor air conditioning systems, using an Optimum Stage-Up Process. This process is programmed into a controller as an algorithm, to provide a process for fast compressor start. This process shortens the time to initiate operation of compressors in a multi-compressor air conditioning system required to meet the demand call under any load condition, and hence shortens the time required for the actual sensed interior region air temperature to reach the interior region temperature set point. The Optimum Stage-up Algorithm estimates the number of compressor stages or steps that must be initiated, based on sensed or measured values, to meet the demand at any load condition. These measured values include the sensed temperature of the interior region being cooled, which is compared to the temperature set point of this interior region as well as measured mixed air temperature and supply air temperature.