A device and method for converting electric energy into pneumatic energy and vice versa, which involves: pumping a liquid forming a liquid piston into a conversion chamber in which a quantity of air is trapped until the air reaches a pressure of a compressed air storage vessel; or churning a liquid by expanding the compressed air in a conversion chamber which is filled with a quantity of liquid, in which device or method the pumping or the churning of the water takes place in the same conversion chamber and consecutively in at least two pumping or churning stages, respectively, provided so as to operate in different pressure ranges. The present device and method can be used in particular in the field of converting and storing electric energy.