The inertia of a running conventional power station is huge, and lowering or raising output is an expensive business. Unfortunately, the demands of electricity consumers are not spread out evenly; and demand raises and lowers in a 24-hour cycle, as lights are turned on in the evening for example.
A method of electricity storage, possible in hilly areas is Pumped Storage. Two lakes are required, at different heights, and the the pumped-storage station between them contains motorized pumps as well as generators.
At times of low electricity demand, the pumps are used to send water from the lower lake to the upper, thereby converting electrical energy via kinetic energy into potential energy. At times of high demand, the water is allowed to flow back to the lower lake, through the generators. This reverses the process, and produces electricity. This extra power can be fed back into the power grid.
The net effect is that no power is generated, and some is lost as heat and noise during the pumping and generating processes. However, the huge benefit is that the difference between low and high demand can be easily handled without having to alter the output of the conventional power stations. The inefficiencies in the pumped storage process are minimal compared with the alternative.