Glossary
Many of the terms below are formally defined in the Balancing and Settlement Code. The definitions here are practical ones, sufficient for interpreting the data on this site.
Acceptance Stack (NESO Skip Rates)
In the context of NESO skip rates, the stack of accepted orders which define the Imbalance Requirement.
All BM (NESO Skip Rates)
A skip-rate calculation method which keeps system tagged acceptances in the Acceptance Stack and Feasible Merit Stack
Bid
Any order to buy a commodity. In the context of balancing mechanism, an order to reduce generation or increase demand.
BM (Balancing Mechanism)
The market-based mechanism in which NESO balances the electricity system in real time.
BM Unit (Unit)
A unit identifier that participates in BM actions; belongs to a Facility.
BOA (Bid Offer Acceptance)
An acceptance of a bid or an offer by the System Operator in the Balancing Mechanism.
BSAD (Balancing Service Adjustment Data)
Non-BM balancing trades carried out by NESO. These include interconnector trades, non-BM fast reserve and system trades.
Commercial Skip Rate (CSR)
An asset-based interpretation of skip rates, in which all orders which were commercially in merit are included in the calculations. Intended to complement the NESO Skip Rate calculations.
Direct Exclusion (NESO Skip Rates)
Orders being removed from skip rate calculations at a given stage, because they meet the exclusion criteria of that stage.
Displacement (NESO Skip Rates)
Orders being removed from skip rate calculations because system tagged orders are forcibly added to the In Merit stack.
EFA Date
GB Power is traded according to EFA Dates which start at 2300 local time. This is equivalent to the same time period of a calendar day in continental Europe.
Embedded Generation
Generation connected at the distribution rather than transmission level. Not visible to NESO for dispatch.
Exclusion Stage (NESO Skip Rates)
A stage in the NESO skip rate calculations which removes orders either from the Acceptance Stack or the Feasible Merit Stack.
Facility
Essentially a power station or interconnector. An aggregation used within GenCharts to represent a physical site containing one or more BM Units of the same fuel type.
Feasible Merit Stack (NESO Skip Rates)
The stack of all orders, accepted or unaccepted, which are eligible for inclusion in a given stage in NESO's skip rate calculations.
FMS (Final Metering System)
The half-hourly metering system used in settlement. Differs from OMS, the real-time operational metering.
Half Hour
The half hours used in the dropdowns start at 2300, because they are the component half hours of the corresponding EFA Date.
Headroom
A measure used by NESO to assess how much power is available in the BM. We have calibrated our calculations against examples provided publicly by NESO.
Imbalance Requirement (NESO Skip Rates)
In the context of NESO's skip rates, the MWh sum of the orders in the Acceptance Stack at a given stage, used to determine which orders are deemed In Merit.
Indirect Exclusion (NESO Skip Rates)
Orders being removed from skip rate calculations at a given stage, because other the exlusion of other accepted orders reduces the Imbalance Requirement.
In-Merit (Bid/Offer)
A bid or offer that is economically competitive and could be accepted by NESO in the Balancing Mechanism based on price. In the context of skip rate calculations, there are 5 levels of processing where bids/offers that appear in-merit based solely on price may be filtered out due to operational constraints (e.g. long notice periods, ramping to SEL/SIL, transmission constraints).
In-Merit (NESO Skip Rates)
A bid or offer from the Feasible Merit Stack which, when ordered by cost, falls below the Imbalance Requirement.
In-Merit Stack (NESO Skip Rates)
The stack of orders after volume adjustements to meet the Imbalance Requirement and exclusion criteria from the calculation method PSA or All BM
INDO (Indicated National Demand Outturn)
A measure of consumer demand on the grid, the 'natural' interpretation of demand. This sums metered generation and interconnector imports to estimate National Demand i.e. ITSDO less the demand used for power generation. Calculated from OMS data. Includes generation from Embedded Generation.
ITSDO (Indicated Transmission System Demand Outturn)
A measure of total load on the grid. This sums metered generation and net interconnector flows, pumped storage demand and estimated station transformer load. Calculated from OMS data.
MEL (Maximum Export Limit)
The declared availability of a given BM Unit at a point in time.
MIL (Maximum Import Limit)
The declared availability for imports of a given BM Unit at a point in time.
Net BOA
The net sum of BOA values for a given aggregation of settlement period, unit, fuel etc.
NESO
The National Energy System Operator. The public body tasked with balancing an increasingly complex grid as renewables, batteries and distributed generation change how the system works.
NESO Skip Rate
A system-operator view of skips. Estimates how the system would balance in a perfect world with no skips, and which orders would be in merit in that world. Treats unaccepted orders in this subset as skipped. This is a good metric for NESO to monitor its overall dispatch performance. Operators wanting to understand why their orders were not accepted may prefer the asset-based Commercial Skip Rate.
Offer
Any order to sell a commodity. In the context of balancing mechanism, an order to increase generation or decrease demand.
OMS (Operational Metering System)
Real time data fed to NESO/Elexon which among other things allows the reporting of INDO, ITSDO.
PN (Physical Notification)
A declared physical generation/demand level for a BM Unit at a given time.
PSA (NESO Skip Rates)
A 'Post System Actions' skip-rate calculation method which removes system tagged acceptances from the Acceptance Stack and Feasible Merit Stack
SEL (Stable Export Limit)
The level at which generation is considered stable. Limits BOA availability - unit will not be accepted to run at less than SEL.
SEM (Single Electricity Market)
The Single Electricity Market for the island of Ireland (Republic of Ireland and Northern Ireland).
SIL (Stable Import Limit)
The level at which imports are considered stable. Limits BOA availability - units will not be accepted to import less than SIL. Expressed as a negative value.
Skipped (Bid/Offer)skipped
A bid or offer that was in-merit but was not accepted. There are many reasons why units might appear skipped e.g. constraints, provision of ancillary services, dynamic data.
Abbreviations
| Abbreviation | Meaning |
|---|---|
| CCGT | Combined Cycle Gas Turbine |
| OCGT | Open Cycle Gas Turbine |
| Recip | Reciprocating Engine (gas or diesel peaker) |
| IC | Interconnector |
| PS | Pumped Storage |