Services that ensure the reliability and safety of the electricity supply system. Electricity market operators purchase ancillary services to control frequency and maintain system voltage within precisely specified limits during sudden and unexpected changes in supply or demand, and to allow for ‘black start’ in the unlikely event of total system failure. Market rules allow users to provide ancillary services, but most of these services must be provided at extremely short notice (within seconds) or are otherwise beyond the capability of end users’ installations. The cost of ancillary services is passed to energy retailers by the market operator and recovered from end users through a component of their retail energy bills.