Transmission Network Use of System
The charge for using the high-voltage national transmission grid, recovered from suppliers and ultimately from customers.
TNUoS charges (often written TUoS in customer contexts) are the cost of using the high-voltage national transmission system — the grid of pylons and substations that moves electricity at 275 kV and 400 kV from generators to the regional distribution networks. The transmission system is operated by the National Energy System Operator (NESO, formerly part of National Grid ESO) and the asset owner is the relevant transmission owner (National Grid Electricity Transmission in England and Wales; SP Transmission and Scottish Hydro Electric Transmission in Scotland).
Historically, large customers paid their TNUoS share via the Triad mechanism — three half-hour periods during the highest-demand winter evenings each year, which determined the customer's share of transmission cost recovery for the following year. Since the Targeted Charging Review took effect, the residual element of TNUoS for non-domestic customers has been recovered through fixed bands based on a customer's historic agreed capacity and net volume, removing the original Triad-avoidance incentive on the residual element.
The forward-looking element of TNUoS — the locational signal that reflects where on the network you are connected — remains, but is significantly smaller. For practical procurement, TNUoS is now a largely fixed cost that depends on the customer's TCR band rather than on real-time behaviour.
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Targeted Charging Review
TCROfgem's reform of how residual transmission and distribution network costs are recovered; replaced volumetric Triad and DUoS residual charges with fixed bands.
Read definitionTriad
The three highest-demand half-hour periods on the GB transmission system each winter; historically determined large customers' transmission cost share. Largely superseded by the Targeted Charging Review.
Read definitionDistribution Use of System
DUoSThe charge electricity suppliers pay to regional Distribution Network Operators for using local distribution wires; recovered from customers through the unit rate.
Read definitionBalancing Services Use of System
BSUoSThe charge that recovers the cost of balancing the national transmission system in real time; restructured in 2023 to be paid by generators only on the demand side.
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