Mankind is forced to respond to the signals that nature gives: the winter is broken into pieces, swathes of land turn into deserts, lakes dry up etc., etc. The energy that people need must come only from renewable sources. Mankind is forced to rely only to renewable sources for as little expense as possible. Electricity generators and transmission lines must have increased capacity to cover the needs of mankind, taking into account the unpredictability of nature, the storage device must have sufficient volume to supply customers at the renewables absence. The energy that is not covered locally must be bought from other energy systems, but in such a quantity that the price for consumers is the lowest. An opportune ratio between locally produced and purchased electricity depends on many factors and is the main topic of this article.