Transmission Network Use of System (TNUoS)

The TNUoS charging regime is slightly complicated for batteries. It depends on if the site is transmission- or distribution-connected, where it is, and what it does during Triad periods.

Generally, if you import during a Triad, you'll pay.

If you export during a Triad, you might get some revenue depending on where the site is, according to Embedded Export tariffs.

If you're a site bigger than 100MW or connected directly to the transmission network, you also face a fixed cost Wider Generation tariffs.

In the forecast, we model TNUoS due to Embedded Export tariffs only. These can be sizeable and are a non-fixed cost.

TNUoS Overview

Grid connection	Import (demand)	Export (generation)
Distribution	Fixed cost: £0 for batteries due to final demand exemption Flexible (Triad) cost: £ avoidable, (and smaller than it used to be) as per network rates	Fixed cost: £0 Flexible (Triad) cost: £ avoidable, as per Embedded Export rates. £0 for Scotland, Northern, North West & Yorkshire
Transmission or embedded > 100MW assets	Fixed cost: see export Flexible (Triad) cost: £ avoidable, (and smaller than it used to be) as per network rates	Fixed cost: £+/- set by Wider Generation tariffs, and annual load factor (ALF) calculation Flexible (Triad) cost: £0

Batteries connected to the distribution network are exempt from TNUoS fixed import fees (provided they submit a Non-Final Demand form to NGESO).

All batteries (distribution or transmission) are exposed to flexible TNUoS charges if they import during a Triad, according to import rates (Figure 2 here). Distribution-connected assets get paid flexible TNUoS rates depending on where they are and their average power over Triad periods, according to Embedded Export Tariffs (shown below).

Transmission-connected assets are subject to fixed-cost TNUoS charges as per the Wider Generation tariffs. They can be positive or negative. Annual load factors are used in the calculation of these charges. For batteries without three years of historical data, the generic Annual Load Factor (ALF) is 1.2391% (source).

Embedded Export Tariffs vary from region to region

Depending on the Grid Supply Point (GSP), the amount that distribution-connected batteries are paid for exporting during Triads varies. We model Embedded Export tariffs using ESO's five-year regional Transmission Network Use of System (TNUoS) charges. We assume that they remain stable from year 6 onwards.

Below, we show these tariffs for the next three years.

We assume 100% capture rate for Distribution connected batteries

These tariffs only apply to distribution-connected batteries with a capacity below 100MW. We model their Triad revenues assuming 100% capture rate. In other words, assuming they are exporting at full power over the Triads. You can apply your capture rate to these numbers.

We assume no site imports during triads

We assume sites don't import during Triads - so you don't pay import flexible TNUoS (which everyone, distribution or transmission connected, pays if they import). If the site does import during Triads, it can be expensive.

Transmission-connected batteries pay fixed TNUoS

Export rates for transmission-connected generation have shifted from time-of-use (via the Triad mechanism) to a fixed rate via the Wider Generation tariffs. This is a result of the Targeted Charging Review.

The rate can be positive or negative and varies throughout the country. See Figure 4 here.

For example, let's model a transmission-connected battery site in South Wales, due to come online during 2025

Transmission-connected batteries fall into the 'Convention Carbon' generating class for TNUoS calculations.

The calculation for the Wider TNUoS tariff, Conventional Carbon, is:

Taking numbers from the 5-year view of TNUoS tariffs, in the year 2025/2026 (p.14) and assuming the site has no historical data, we use an Annual Load Factor (ALF) generic for storage of 1.2391% (p.71).

This gives the calculation:

2.349244 + (- 8.341082 x 1.2391%) + (0 x 1.2391%)+ - 2.167565 £/kW = 0.078325 £/kW (to 5 significant figures), or £78.33/MW/year.

The site will be charged £78.33/MW over 2025 in TNUoS fees, plus the variable rate for anything it imports during triad periods (which would hopefully be negligible).

Our forecast for 2025 shows a site in South Wales earning ~£85k/MW in 2025. The TNUoS fee, therefore, constitutes a cost equivalent to 0.1% of annual revenues.

Note: Fixed TNUoS fees are charged in £/kW, and the kW rating is as the site is listed in the TEC register. The maximum capacity is taken if the site size listed in the TEC register changes during the year.

How does this look in Scotland?

In North Scotland (the most expensive of the Scottish demand regions), the calculation looks like:

2.747082 + (27.563546 x 1.2391%) + (5.142500 x 1.2391%) - 1.296387 = 3.13938 £/kW or £3,139/MW/year.

In 2025, our forecast shows this equates to around 5% of the site revenues - so more significant.

The Modo Forecast revenues do not include fixed network fees

In the same way we do not include fixed charges like standing charges, FIT or RO line items on the electricity bill, we don't include fixed TNUoS rates in the forecast line items, like Wider Generation tariffs. These should be considered in the OPEX costs of the site.