A 300 l 300 bar air tank contains 90 m3 perfect gas and about 81 m3 of air due to Van der Waals forces It means about 13 kW 47 MJ of energy assuming isothermal compression expansion W P2V2 x Ln P1 P2 A multi stage 300 bar scuba air compressor Coltri MCH13 for instance provides an air throughput of 13 m3 h 215 l s with a 4 kW electric motor Hence it takes 81 13 6h 12mn to fill the 300 l air tank at 300 bar Assuming a continuous 4 kWh power consumption during 6H 12mn 25 kW input energy Resulting energy conversion efficiency 13 kW 25 kW 52 much better than the 15 given in the article Practically the real electric consumption is a little bit lower due to the fact that at low air pressure first compression phase the compressor uses less than 4 kW For a Coltri MCH13 ET description see Abaca