This term denotes a collection of secondary services offered to help insure the reliability and availability of energy to consumers. It generally refers to any service related to provision (but not generation) of energy other than the use of transmission facilities. These services are typically offered by energy utilities and support companies and sold to wholesalers, producers and consumers. These services may include supplementary energy generation during peak demand periods, demand-side management (DSM), maintenance of replacement reserves, management of control systems that maintain steady system voltage, and a host of others.
In practice, ancillary services typically refers to services needed to shift a large-scale customer (e.g. a municipality or industrial consumer) from one energy supplier to another when the need arises, and they are usually (but not always) required as part of any well-rounded energy package negotiated with a large-scale consumer. Some types of ancillary services may be required by regional or national regulations when providing service for certain types of customers.
Ancillary services are not the same as value-added services.
See also:value-added services, transmission, WSCC, National Energy Regulatory Commission, wholesale