Example energy storage systems (20, 20′, 20″) comprises a fluid circuit (22, 22′, 22″) and an electrical unit (24, 24′, 24″) configured to operate as a motor in a first phase of operation and to operate as a generator in a second phase of operation. The fluid circuit (22, 22′, 22″) comprises a first fluid container (30, 30′, 30″) situated so content of the first fluid container experiences a first pressure level; a tank (32, 32′, 32″) having its content at a second pressure level (the second pressure level being less than the first pressure level): and, a first hydraulic motor / pump unit (34, 134, 34″connected to communicate a first working fluid between the tank and the first fluid container. In the first phase of operation electricity is supplied to the first hydraulic / motor unit (34, 134, 34″) whereby the first hydraulic / motor unit transmits the first working fluid from the tank into the first fluid container (30, 30′, 30″). In the second phase of operation pressurized first working fluid in the first fluid container (30, 30′, 30″) is transmitted from the first fluid container through the first hydraulic / motor unit 34, 134, 34″) to the tank (32, 32′, 32″), thereby causing the electrical unit (24, 24′, 24″) to generate electricity.