As used for electricity, this usually refers to the entity that is the system operator for the high voltage electricity transmission system. An SO is responsible for balancing the electricity system by ensuring that generation matches demand. In Great Britain, before 1 April 2019, this was National Grid Electricity Transmission plc (NGET) with a combined system operator (SO) and transmission owner (TO) role. NGET legally separated its TO and SO roles from 1 April 2019, transferring the SO functions to National Grid Electricity System Operator (NGESO) (see Practice note, National Grid: splitting system operator and transmission owner roles). Distribution network operators (see Distribution network operator (DNO)) may need to take on a system operator role in the future, to become distribution system operators (DSOs). See also Transmission system operator (TSO).

As used for gas, this usually refers to the entity that is responsible for balancing the inputs and offtakes from the high pressure gas pipeline network (the national transmission system (NTS)). In Great Britain, the gas SO is National Gas Transmission (NGT). For more information, see Practice note, Downstream gas industry: overview.