A hydropower device for generating electric power comprises a pipe having an upstream portion and a downstream portion and being arranged to convey water. A valve is provided at the downstream portion and adjustable between a closed position hindering the water and an open position permitting the water to flow. A cylinder member defines an inner space and having a first end and a second end. The inner space extends away from the pipe along a centre axis (x) and receives water from the pipe via the first end when the valve is in the closed position. A piston is disposed in the inner space and movable back and forth along the centre axis. The piston moves away from the pipe by means of an overpressure arising when the valve is in the closed position.