A water reservoir system for generating, accumulating, storing, and releasing electrical energy comprises a reservoir wall built in a shallow body of water such as a sea or an ocean with a height exceeding the outside water level by about 10-25 m, thereby defining an interior of the water reservoir. Excess electrical energy from other renewable sources of electricity such as wind, solar power, or supplied by a local power grid is used to operate water pumps to fill the interior of the water reservoir with water during times of peak supply of electricity. Water is drained from the water reservoir to the outside body of water and generates electrical energy by flowing over a plurality of water turbines, thereby generating electricity and supplementing electrical power for the local power grid during times of high demand. Additional interior sources of renewable energy may be used to supplement external sources of electrical power in operating the system of the invention.