Install a Compressed Air Flow meter on Air Outlet, an Electrical Meter and an Hour Meter. Sit down, chart all data, make your graph and you will arrive at the efficiency.
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I not only use all the brains that I have, but all that I can borrow. Woodrow Wilson
You need to measure mass flow, pressure in and pressure out. Then calculate actual work on the gas as compared to the electric meter. This will give the over effiency of everything from the motor to the bearings to the gas compressor its self.
If you want to know the efficiency of the compressor you need to measure suction temperature and discharge temperature. Then get out the thermo books and fiquire it out.