An intermittent energy source is any source of energy that is not continuously available due to some factor outside direct control. The intermittent source may be quite predictable, for example, tidal power, but cannot be dispatched to meet the demand of a power system. Effective use of intermittent sources in an electric power grid usually relies on using the intermittent sources to displace fuel that would otherwise be consumed by non-renewable power stations, or by storing energy in the form of renewable pumped storage, compressed air or ice, for use when needed, or as electrode heating for district heating schemes.
The storage of energy to fill the shortfall intermittency or for emergencies is part of a reliable energy supply. The capacity of a reliable renewable energy supply, can additionally be fulfilled by the use of latency measures and backup or extra infrastructure and technology, using mixed renewables to produce electricity above the intermittent average, which may be utilised to meet regular and unanticipated supply demands.
The penetration of intermittent renewables in most power grids is low, but wind generates roughly 16% of electric energy in Spain and Portugal, 9% in Ireland, and 7% in Germany. Wind provides nearly 20% of the electricity generated in Denmark; to operate with this level of penetration, Denmark exports surpluses and imports during shortfalls to and from neighbouring countries, particularly hydroelectric power from Norway, to balance supply with demand. It also uses large numbers of combined heat and power (CHP) stations which can rapidly adjust output. The large thermal stores in these systems are also utilised to store surplus wind energy, since thermal storage is the cheapest form of energy storage.