A power reactor (1) for energy transfer comprising a shaped casing (2), which sits upon a support structure, and a winding (3) which is electrically connected to an electrical energy supply network and is contained inside the shaped casing (2) with which it is associated through support means (4). The shaped casing (2) and the winding (3) are arranged a first distance (D) apart which is a function of the electrical current, of the inductance and/or of the geometry of the winding and is not less than a predetermined minimum value in order to make it possible to absorb the energy losses created by the parasite currents generated by the magnetic flux produced by the winding (3) and that engages the shaped casing (2), the first distance (D) diverging towards the shaped casing (2) from the end portions (3a, 3b) of the winding (3) crossed by the flux lines of the magnetic field that link together with the winding (3).