Transmission Network Use of System (TNUoS) costs explained

The information in this video and article was accurate as of 1 April 2025. We update our TPCs Explained hub every Spring to reflect any changes.

Third Party Costs Explained: Transmission Network Use of System (TNUoS)

The TNUoS charge recovers the cost of maintaining and upgrading Great Britain’s onshore and offshore transmission system. It also includes any support for cap-and-floor supported subsea interconnectors with other markets.

How does TNUoS work?

The transmission system operates at very high voltages. It’s responsible for flowing bulk flows of power from the transmission system to local distribution networks, where most end customers are located. It’s also responsible for flowing power to and from interconnectors with other countries.

How much of your energy bill does TNUoS account for?

In 2025, the charge is expected to account for around 5% of the electricity bill for a typical low-voltage, site specific customer. This figure makes it the fourth largest third-party cost. However, TNUoS will vary by location and customer type.

How are TNUoS costs recovered?

This charge covers the maintenance and upgrading of infrastructure, including power lines, substations, and transformers that operate at high voltages. These high-voltage lines are the ‘motorways’ of the energy system. A few very large power consumers are connected at this level.

The transmission network across England, Wales, and Scotland includes extremely high-voltage lines carrying either 400kV or 275kV. In Scotland, the 132kV network is also classed as transmission.

How is the TNUoS fee structured?

The charge is set by the transmission network operators and depends on the users’ location and energy consumption over peak periods. For example, areas farther away from the bulk of power stations may have higher charges due to the cost of transporting electricity over long distances.

Looking at TNUoS as a proportion of all network costs, it accounts for a significant 18%. This covers the high-voltage power lines and other infrastructure, including the wind farm connectors and undersea bootstrap cables.

How do the TNUoS usage fees work?

TNUoS includes a substantial fixed element, and a pence-per-day charge that varies according to customer size and connection voltage. There’s also a much smaller usage-based charge that varies from zero in Scotland and the north of England to the highest charge in London and the south-west.

This variation occurs because the north generates the most electricity, but the south uses more, so the Grid tends to flow power southwards. Currently, the East Midlands region doesn’t pay, but National Grid forecasts this will change in 2027.

For half-hourly customers, the charge is multiplied by their usage over the so-called ‘triad periods’. For non-half-hourly customers, the charge is based on a forecast of their weekday usage between 4pm and 7pm over the winter period.

Will you know about TNUoS costs in advance?

Yes - the National Energy System Operator (NESO) publishes the TNUoS tariffs annually by 31 January, which then take effect from 1 April. It also publishes forecasts of the upcoming charge throughout the year.

While we know the charge, each customer’s consumption is only forecast. Therefore, depending on your tariff, there may be a period of reconciliation between the forecasts and actual usage once the winter period is over.

What drives the TNUoS charge?

The main drivers of this charge are:

• The low carbon transition and construction of power stations in remote locations
• Regulatory changes, either through regulatory intervention or code governance changes, can lead to changes to the Connection and Use of System Code (CUSC). CUSC outlines how the charges are recovered
• Changes to applicable demand and meter numbers

To learn more about the other TPCs in your energy bill, head to our TPCs explained hub using the button below. Or, to understand more about electricity prices, download our latest bi-annual Electricity Prices Explained guide.

Disclaimer

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