The present invention relates to an energy storage apparatus (10) comprising a compressible spring (11) wound around a main shaft (28) extending between a releasing means (20) and a compressing means (21), said compressing means (21) and releasing means (20) being in a fixedly attached relationship with said compressible spring (11) at two ends of the main shaft (28), said compressing means (21) and releasing means (20) being rotatable at different speeds so as to compress the compressible spring (11) at one longitudinal side of the main shaft (28) while simultaneously releasing the compressible spring (11) at the other longitudinal side, said energy storage apparatus (10) further comprising a hydraulic jack (26) continuously driven by a hydraulic pump (13) in the manner that said compressible spring (11) is compressed by said compressing means (21) to which rotational movement is imparted by said hydraulic jack (26).