Unidentified Gas (UIG)

Understand what UIG is, how it’s calculated and charged, and how you can help reduce UIG.

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    What is Unidentified Gas (UIG)?

    The majority of gas consumed in Great Britain can be accounted for as it is metered and registered. However, some gas is lost from the system, or not registered, due to theft, consumption by unregistered supply points and other reasons.

    The gas that is off taken from the Local Distribution Zone (LDZ) system, but not attributed to an individual Supply Meter Point or accounted for as Shrinkage, is referred to as UIG.

    There are aspects of UIG that is ‘temporary’ (i.e. at D+5 allocation) which can be caused by factors such as LDZ or DM measurement errors or NDM Demand Modelling error. This temporary UIG will remain until such time that meter reads are received when the actual consumption is revealed. Post reconciliation, the actual UIG will start to be revealed.


    How is UIG calculated?

    UIG is calculated in Gemini using the following formula:

    UIG = Total LDZ Energy – DM Energy – NDM Energy – Shrinkage

    This is calculated on a daily basis and is reported against a UIG meter in Gemini. UIG is calculated per LDZ and then shared out based on Class and End User Category (EUC).

    • Total LDZ Energy is the energy entering an LDZ from the NTS. This is recorded at multiple gas input points and includes an adjustment for stock change for the day (change in the amount of gas retained within the pipes).
    • Total DM Energy is the total energy for Class 1 & 2 sites based on reads received from Shippers and Daily Metered Service Providers (DMSP)
    • Total NDM Energy is the total energy for Class 3 & 4 sites based on an estimate using the NDM Algorithm – (see Nominations & Allocations for more detail)
    • Total Shrinkage is a fixed energy figure calculated by Network Operators, based on expected losses

    The UIG Allocation Issues Tracker provides a record of known issues with LDZ Allocation which has impacted UIG levels. This can help customers understand any unusual spikes and dips in UIG when performing analysis. You can download the latest version below.

    How is UIG shared out?

    Due to the changes in gas settlement Project Nexus brought about, the industry agreed there was a requirement to fairly apportion the total UIG between classes and End User Category (EUC).

    To help with this, an independent expert (AUG Expert) helped the industry develop a methodology and provide a table of weighting factors that assigns the correct amount of UIG to different classes of meter points.

    The table of weighting factors is used in the daily gas nomination and allocation processes.

    Daily measured or estimated gas throughput in each LDZ is weighted using the AUG table factors to assign daily UIG to Shippers based on their throughput by meter point class and EUC.

    What happens after Allocation?

    Meter Reads are used to reconcile the consumption of each individual Meter Point, with the System Average Price (SAP) rates used to calculate the monetary value.

    Note: The System Average Price (SAP) can be found on National Gas Transmission’s data portal

    The equal and opposite amount of all individual reconciliations’ energy (and monetary value) amends the Unidentified Gas (UIG) position. Those Reconciliations appear on the monthly Amendment Invoice.

    UIG Reconciliation is shared out based on the latest (reconciled) allocations or measurements for the last 12 billing months, using the applicable UIG Weighting Factors.

    Meter point reconciliation, both Daily Metered and Non-Daily Metered, can be processed back to the Line in the Sand (i.e. 1 April minus three years).

    However, all the meter point reconciliations back to that date for a month are summed up, and all charged/credited to UIG. There is no gain or loss of energy in the process.

    The sum of all the meter point reconciliations is divided by 12 and shared back across 12 months of weighted throughput.


    How you can help reduce UIG

    Within the daily Demand Attribution calculation, there are various components that can each contribute to UIG, for example LDZ or DM measurement errors and NDM modelling error.

    These causes can be mitigated or reduced by actions taken by the Gas Industry, including:


    • Reviewing accuracy of AQs and adjusting where required
    • Promptly registering Shipperless/unregistered sites
    • Supplying regular accurate monthly reads, in line with read frequency, for NDM
      meter points
    • Submitting notifications of meter asset exchanges/updates and/or market sector code changes promptly
    • Supplying accurate DM Nominations, as early as possible each day
    • Using the Class 2 product for larger NDM LSP sites where appropriate and submitting reads as per UNC obligations
    • Support NDM Demand Estimation modelling by providing additional sample data to Xoserve, especially for small LSP markets
    • Continuing to be diligent in managing consumer theft of gas

    Gas Transporters

    • Reviewing accuracy of LDZ offtake equipment to minimise errors


    Reconciliation by Month Report

    We introduced the Reconciliation By Month Report after the implementation of our updated UK Link System, also known as Project Nexus. This report details Post-Nexus Reconciliation for all class types split by the original billing month, to which the reconciliation relates by LDZ/EUC and product class. Associated financial values are also provided, alongside assurances on the functionality of the UIG process.

    The total reconciliation energy returned within the report will agree to the reconciliation energy issued on the corresponding month’s Amendment Invoice.

    More information

    More information on data that is available to the industry to support analysis and improve understanding of UIG calculations can be found below:

    There is also an overview of UIG in our online UIG e-learning material.

    Information relating to the Allocation of Unidentified Gas Expert (AUGE),and the latest Weighting Factors is available on the Joint Office of Gas Transporters website.


    Common queries

    UIG only applies to Local Distribution Zone (LDZ) sites.

    Non-Standard sites on Local Distribution Zones (LDZs) will be allocated a Class and End-User Category (EUC) based on site attributes and will attract the appropriate UIG Weighting, based on the prevailing set of Weighting Factors.

    The Registered System User’s (RSU) total system outputs include their share of Local Distribution Zone (LDZ) UIG.

    The RSU will not be billed for their UIG allocation on the Network. They will only be billed for any excess or shortfall in the gas that they have delivered to the total System compared to total outputs. This is a System-wide calculation, not LDZ by LDZ.

    This means that the RSU share of UIG will not appear on any invoice. Instead, it will be just another input to the existing Energy Balancing process. As UIG meters are part of the overall daily imbalance (they are within the allocated outputs of your overall daily imbalance) - timings, prices etc are unchanged.

    As stated above, UIG is not cashed out or billed separately. Imbalances prior to D+5 are not chargeable – it’s only the D+5 closed-out position that feeds into Energy Balancing. The overall daily imbalance is cashed out at the relevant System Marginal Price (SMP).

    There are no Scheduling Charges on UIG or Non-Daily Metered (NDM) allocations, so no penalty or incentive to buy the exact quantity as per the Nominations.

    This is explained further in our Unidentified Gas Education Pack.

    Unidentified Gas (UIG) can be re-calculated up to four times a day for the day ahead (‘D-1’) and up to five times within-day (for ‘D’) whenever there has been a change to a Daily Metered (DM) Nomination or the forecast Composite Weather Variable (CWV).

    Unidentified Gas (UIG) could be negative for one or more Local Distribution Zones (LDZs) for any given Gas Day. This is not necessarily an error, as UIG at D+5 is only an interim accounting position and subject to further reconciliation.

    Where negative UIG occurs for a number of LDZs this is likely to be as a result of NDM modelling error e.g. where the estimate of NDM demand is too high for the day meaning UIG goes negative (as the balancing figure). The NDM Modelling Error presentation provides more detail .

    Currently, total Unidentified Gas (UIG) is not visible on Shipper screens in Gemini (Shippers only see their own share).

    It can be viewed on a spreadsheet (both energy and %) on our website, which is updated once a week.

    You can access this spreadsheet in UK Link Secured Documentation *

    (Then 18. NDM Profiling and Capacity Estimation Algorithms/Demand Estimation Project Nexus/LDZ UIG Values)

    *Access to secure links are only available to gas shippers and transporters. To gain access find out about accessing secure documents.

    Yes, depending on the accuracy of the original measurements and estimates used in Allocation, UIG Reconciliation may be a debit (charge) or credit (refund).

    It takes many months or years for all meters to be read and trigger reconciliations.

    This means that the monthly positions will keep changing and may switch between debit and credit over time.

    The net charge for UIG Reconciliation each month reflects the equal and opposite of the net effect of all the primary Meter Point Reconciliation processed each month.

    As of March 2022, there are over 170,000 sites which have not had a meter reading since the current “Line in the Sand” (the Code Cut-off Date of 1 April 2019), so there is still scope for further Reconciliation movements.

    The source of the Reconciliations/amendments each month can be seen in our ‘Reconciliation by Month’ report. You can find the report under ‘Useful Links’ at the bottom of our Unidentified Gas (UIG) webpage.

    The Industry expects net Unidentified Gas (UIG) to end at a lower value however, it may not move steadily towards that figure.

    Instead, there may be peaks and troughs on the way to the final position, again depending on the individual transactions that are processed each month.

    The Unidentified Gas (UIG) smear is calculated as a flat smear across all the qualifying months, rather than being smeared back to the months to which the primary reconciliations relate.

    For example, if the primary Rec quantities in a Local Distribution Zone (LDZ) totalled 12m kWh debit:

    • Monthly UIG smear: +12m kWh / 12 = 1m kWh credit per month.

    The monthly UIG would be shared out in line with the latest weighted throughputs.

    Now that the Post-Nexus arrangements are fully mature, the UIG would always be smeared out in 12 equal amounts, regardless of the relevant months of the underlying transactions, which could be longer or shorter.

    The aim of the Reconciliation by Month Report is to show which original months the UIG relates to, which can then be combined with information of the original UIG allocations, to be able to assess whether UIG is declining or increasing as meter point reconciliations take place.

    There are two online screens and one Application Programming Interface (API) through which Shippers can view both Forecast and Allocation Unidentified Gas (UIG) Quantities.


    Shipper Forecast UIG Report

    Shipper Allocation UIG Share Report 

    View Shipper UIG Values

    It’s difficult to forecast UIG with great certainty because as the ‘balancing figure’ each day, UIG can be influenced by many different factors including:

    • Accuracy of Total Local Distribution Zone (LDZ) Energy forecasts/measurements/stock change
    • Accuracy of DM Energy forecasts/measurements
    • Accuracy of Non-Daily Metered (NDM) Demand forecast which can be impacted by:
      • Annual Quantity (AQ) accuracy
      • Unexpected reactions to weather by Non-Daily Metered (NDM) sites
      • NDM sites in an inappropriate End-User Category (EUC) (e.g. not in a WAR Band EUC)
      • Imperfections in the NDM demand models or supporting sample data
      • Accuracy of forecast/actual weather data
    • Levels of incorrect theft in the LDZ
    • Missing sites in the LDZ, e.g. shipperless or unregistered sites

    All these influencing factors can mean that at D+5 Exit Close-Out, UIG may be negative. UIG will subsequently be subject to Reconciliation, as individual Meter Point Reconciliations occur.

    Note: D+5 = the fifth day after the end of the Gas Day which is when Daily Allocations are finalised (Exit Close-Out in Uniform Network Code terminology).

    UIG webinar material

    The presentation and Q&As from the UIG webinar hosted on 29 June 2022.

    UIG webinar material

    National UIG charts

    You can view our national UIG charts on the links below. 

    Contact us

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