A method for analyzing, monitoring, optimizing and comparing energy used for producing a unit of mass or volume of compressed gas, so called Specific Energy Consumption, in relation to a common output flow in a multiple compressor system, said method comprising: - collecting measured data of common output flow and energy/power use and calculating the specific energy consumption in the multiple compressor system, - identifying which data points of measured specific energy consumption that affiliate to a certain compressor or compressor combination in the multiple compressor system and operating mode(s) of the multiple compressor system; and - plotting the data points of measured specific energy consumption that affiliate to a certain compressor or compressor combination in the multiple compressor system and operating mode of the multiple compressor system and marking affiliation of said data points to the certain compressor or compressor combination and operating mode, wherein the method further comprises constructing the ideal specific energy consumption curve(s) for any combination of the compressors in the multiple compressor system and for the whole range of flow values in demand, and for each total flow amount the ideal specific energy consumption curve show the lowest attainable specific energy consumption at a certain pressure level, wherein the ideal specific energy consumption curve(s) is plotted and each of them is complemented with another curve visualizing the working limit for each certain compressor or compressor combination in the multiple compressor system, and wherein the curves together form a working area for each certain compressor or compressor combination in the multiple compressor system, and wherein data points outside of the working area(s) for each certain compressor or compressor combination in the multiple compressor system are identified and indicated as measuring errors or system or equipment faults. - collecting measured data of common output flow and energy/power use and calculating the specific energy consumption in the multiple compressor system, - identifying which data points of measured specific energy consumption that affiliate to a certain compressor or compressor combination in the multiple compressor system and operating mode(s) of the multiple compressor system; and - plotting the data points of measured specific energy consumption that affiliate to a certain compressor or compressor combination in the multiple compressor system and operating mode of the multiple compressor system and marking affiliation of said data points to the certain compressor or compressor combination and operating mode, wherein the method further comprises constructing the ideal specific energy consumption curve(s) for any combination of the compressors in the multiple compressor system and for the whole range of flow values in demand, and for each total flow amount the ideal specific energy consumption curve show the lowest attainable specific energy consumption at a certain pressure level, wherein the ideal specific energy consumption curve(s) is plotted and each of them is complemented with another curve visualizing the working limit for each certain compressor or compressor combination in the multiple compressor system, and wherein the curves together form a working area for each certain compressor or compressor combination in the multiple compressor system, and wherein data points outside of the working area(s) for each certain compressor or compressor combination in the multiple compressor system are identified and indicated as measuring errors or system or equipment faults.