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   http://www.airtec.co.nz/products.html Abaca