the entracque power plant is supported by a scheme that also primarily consists of three reservoirs. the lower reservoir used by both the chiotas and rovina is lago della piastra. the reservoir is located at the base of the valley and was formed by the construction of an tall gravity dam. its water level has a normal operating elevation of and of its total storage capacity can be pumped up to the upper reservoirs. the pumping is carried out by the power station's nine francis pump turbine-generators, eight belong to chiotas and one to rovina. when either of the upper reservoirs needs to be filled, water is pumped from piastra to the chiotas or rovina upper reservoirs through a series of penstocks and tunnels. this usually occurs during periods of low energy demand, such as at night, when electricity is cheap.