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Transmission Losses

Losses on the Transmission System are allocated across BSC Parties through the use of Transmission Loss Multipliers (TLMs). Transmission Loss Factors exist for each TLF Zone (aligning with the existing Grid Supply Point Groups) for each BSC Season in order to allocate transmission losses on a geographical basis.

Recent changes to TLMs

The CMA has mandated, through secondary legislation and licence changes, the introduction of Transmission Loss Factors for each TLF Zone.

This was initiated by P350 and implemented from 1 April 2018.


Screen shot and link of Transmission Losses video on YouTube

How it relates to you

As energy is transported from the point of production to the end user, some of it is lost. Under the BSC arrangements there are two ways in which these losses are accounted for:

  • Losses on the Transmission System
  • Losses on the Distribution Networks

Losses on the Transmission System are allocated across BSC Parties through the use of Transmission Loss Multipliers (TLMs).

Losses on the Distribution Networks are allocated through the use of Line Loss Factors (LLFs)

Accessing operational data

The Balancing Mechanism Reporting Service (BMRS) is the primary channel for providing operational data relating to the GB Electricity Balancing and Settlement arrangements.

Accessing the TLF data

The ELEXON Portal is where ELEXON upload the associated data.

  • Portal site structure: Operational Data > TLFs determination

Current data

  • Reference Network Mapping Statement (NMS)
  • Load Periods and Sample Settlement Periods
  • Metered Volumes for Sample Settlement Periods
  • Transmission Network Data
  • HVDC Metered Volume Data for Sample Settlement Periods
  • Distribution Network Data
  • Total Delivering and Offtaking Metered Volume Data
  • Nodal TLFs
  • Adjusted Seasonal Zonal TLF (ATLFZs)
  • BM Unit Specific TLFs
  • Seasonal Zonal TLF (TLFZs)
  • Transmission Loss Factor Adjustment (TLFAs)
  • Indicative values of TLMij, for every Settlement Period in the Reference Year with TLFij=0
  • Indicative values of TLMij, for every Settlement Period in the Reference Year with TLFij≠0
  • Adjusted Nodal Power Flows
  • Branch Power Flows
  • Absolute Nodal Power Flows

Historic data

Historic Transmission Loss Multipliers’ Historic Transmission Loss Multipliers data can be downloaded from the ELEXON Portal,

Estimated Transmission Loss Adjustment (ETLMOs)’ Historic Estimated Transmission Loss Adjustment (ETLMOs), are indicative parameters set for the purposes of Balancing Mechanism Reporting Service (BMRS) reporting.

These parameters are the estimated values of the ‘Delivering Transmission Losses Adjustment’ (TLMOj+), and the ‘Off taking Transmission Losses Adjustment’ (TLMOj-). The ISG reviews the ETLMO values annually on behalf of the BSC Panel.

Calculating TLMs

Under the BSC arrangements, TLMs are applied to non-Interconnector Balancing Mechanism (BM) Unit Metered Volumes as a means of scaling to account for losses on the Transmission System.

Interconnector BM Units are assigned a TLM of 1 and are exempt from Transmission Losses.

BSC definition

Transmission Losses on the System is defined in the BSC as the sum of BM Unit Metered Volume over all BM Units (with BM Units that import having a negative value of BM Unit Metered Volume).

The formulas for calculating TLMs are set out in BSC Section T, Paragraph 2.3.1.

There are only two values for TLM per Settlement Period, Zone and BSC Season:

  • one for BM Units belonging to Trading Units that are off-taking in the Settlement Period
  • one for BM Units belonging to Trading Units that are delivering in the Settlement Period

The TLM adjusted Metered Volume for each BM Unit is described as the BM Unit’s ‘Credited Energy’.

In accordance with BSC Section T, Paragraph 2.2, the Transmission Loss Factor Agent (TLFA) determines every year the Transmission Loss Factors (TLFij) for each Zone and for each BSC Season.

Offtaking TLMs

The Offtaking TLM applies to all non-interconnector BM Units that are part of an Offtaking Trading Unit.

Delivering TLMs

The Delivering TLM applies to all non-interconnector BM Units that are part of a Delivering Trading Unit.

Guidance information