An example of an indirect power storage is the pumped storage power plant. Its centerpiece is a large water reservoir, located on a hill. As long as sufficient power is available, water is transported up the mountain with pumps. If electricity is demandes at a later moment, the water runs off and produces the wished energy with a generator. Mainly pumped storage power plants serve as a storage battery to bridge power failures. Gyrating mass storage power plants and compressed air storage power plants work similarly.
Another indirect electricity storage is produced by the conversion of electricity into hydrogen. With the help of the process of the electrolysis hydrogen is generated. To tranform hydrogen back to electricity, the use of fuel cells is demanded. However, the whole process is very elaborate and also goes along with high losses of energy.
Batteries and accumulators store power chemically and are tested sufficiently. But this kind of the indirect power storage is relatively expensive – measured in her low capacities. Today already there are some proven power storages which are functional for the use with photovoltaic systems. Most, however, are not suitable for large amounts of energy. But it is also clear, that for an autonomous use, such as photovoltaic, a power storage is unavoidable.