An air-compressor is rated by manufacturers at their nominal flow-power level. In the commercial leaflets, the flow and the nominal electrical power is published. The nominal power of an electrical motor can be different than the real power consumed. The nominal power only refers to the frame size of the electrical motor. The flow is measured according to the ISO1217 standard.  

The compressor performance is indicated at reference condition, which is the most favourable condition. In reality, during the operating conditions, all parameters defining the real energy consumption, are different than the reference conditions.

The best parameter to categorise different air-compressors is the specific energy consumption at different flow rates !

The specific energy consumption is the amount of Power/Flow. This can be calculated as kW/m3/min or HP/CFM or kW/100CFM.   

 In the US, the CAGI website centralises the various compressors offered in the US, and the datasheets are available for everyone. The CAGI datasheets are mentioning the Specific Input Power at rated capacity and full load operating pressure in kW/100 CFM. Above 15m3/min, a tolerance of +/-5% is allowed.

 In the CAGI datasheets, typical values for 100psi or 7 bar are 16kW/100CFM, 17kW/100CFM,….. In metric units, typical values would be between 5kW/min3/min, 6kW/m3/min and even 7kW/m3/min .

 This value is a meaningful parameter allowing you to compare and evaluate different compressor types at the same pressure.

 When the flow varies from 100% to 0%, the Specific energy consumption will increase !

As the vast majority of compressor units are running in partial load, the specific energy consumption should be published also in partial load, similar to the CAGI datasheets for VSD. 

Have you calculated the specific power consumption of your total installation ?