Slow Reserve
Project Update
We have now shared more information on the key dates for go-live of the new Slow Reserve (SR) service, and details of a requirement to link certain SR windows (positive SR only) for an initial transitional period as we embed the enduring SR service. Please refer to our Slow Reserve Transition Plan for full details.
Reserve is needed for frequency management when there is an imbalance between supply of energy and demand for energy.
When instantaneous supply is not enough to meet the demand, the frequency falls; where supply outstrips demand, the frequency rises. Additional generation or demand is needed to re-establish this balance. Initially, this is provided by Frequency Response which initiates automatically according to system frequency. Reserve is then instructed to replace the energy delivered by frequency response in accordance with system requirements.
Slow Reserve (SR) is primarily aimed at reacting to post-fault disturbances to restore energy imbalances to +/- 0.2Hz within 15 minutes of a loss event (generation or demand). For Negative Slow Reserve (NSR), units are instructed to increase demand or decrease generation in full within 15 minutes. The inverse is true for Positive Slow Reserve (PSR). Slow Reserve is open to any technology with the ability to provide a net change in demand/generation of at least 1MW.
Further enquiries:
Email: [email protected]
How to Participate
Service Terms and Procurement Rules
Service Design and Guidance
To participate in Slow Reserve your asset/unit will need to be registered in the Single Market Platform (SMP).
The Enduring Auction Capability (EAC) Market design Explainer can be downloaded.
Service Providers holding a Slow Reserve contract and wishing to transfers obligations to another unit should complete and submit the request to all of the following addresses :
Please ensure all template instructions are followed.
Please find supporting documentation below to support IT integrations for the Slow Reserve service. Please note, whilst all of the IT Integration material in this section is applicable to non-BM participation, only the Performance Metering documentation is relevant to BM service providers.
OBP Documentation
We have released version 1.3 of the OBP Web Service Specification for non-BM service providers, together with WSDL files that formally define the interface between service providers and the OBP application. The documents specify the Quick and Slow Reserve services, which include:
- Physical Notification service
- Availability service
- Heartbeat service
- Dispatch/Cease Instruction service
In addition to the API documentation, NESO has issued a unified Business Logic Document (BLD) version 1.0 for both Quick Reserve (QR) and Slow Reserve (SR), offering guidance for non-BM providers of both contracted and optional services. The BLD applies equally to QR and SR services. A new validation has been introduced to ensure that declared capacity does not exceed pre-qualified capacity; if it does, NESO will reject it.
The BLD outlines the business rules and exceptions for the API, which Quick Reserve and Slow Reserve service providers must implement. It includes the following:
- Availability Service: Validation NESO applies to information from providers.
- Availability Confirmation: Validation NESO expects providers to apply to information from NESO.
- Dispatch/Cease Service: Validation NESO expects providers to apply to information from NESO.
- Dispatch/Cease Confirmation Service: Validation NESO applies to Dispatch/Cease information from providers.
- Heartbeat Service: Validation NESO applies to information from providers.
- Heartbeat Negative Acknowledgement (NACK): Validation NESO expects providers to apply to information from NESO.
- Physical Notification: Validation NESO applies to information from providers.
- Physical Notification Confirmation: Validation NESO expects providers to apply to information from NESO.
This document describes the operational metering architecture that NESO has in place to enable non-BM participation in Slow Reserve.
This document describes the proposed solution for NESO to accept and store Slow Reserve Performance Metering data from individual Service Providers (SPs). Please note this is applicable to both BM and non-BM participation.
The requirement for Slow Reserve Performance Metering Data for BM and NBM units at a granularity of every 15-seconds however second-by-second metering is also acceptable. The interface data and transmission requirements that service providers must meet are also outlined, describing the concept of operations for the interface, defining the message structure and protocols that govern the interchange of data, and identifying the communication paths from which NESO expects data to flow.
The intended audience of this document are external service providers and internal teams across NESO that are associated with development and operations of the systems connected to the interface.
The data design specification provides the following information:
- a general description of the interface
- data specifications including source, target and validation details, data formats and classification
- a description of the transport characteristics including transfer, data exchange, interface processing requirements, flow control and security
- assumptions and outstanding questions
Also published are two sample files to provide an example of the required data structure, one for 15-second metering files and the other for 1-second metering files.
Technical Onboarding Webinar
Slow Reserve Technical Onboarding Webinar 6 November 2025
Following the webinar held on 6 November 2025, we have shared the slides and a recording of the webinar.
We have now completed our initial proposed service and procurement design for the new Slow Reserve service, incorporating both BM (Balancing Mechanism) and non-BM (non-Balancing Mechanism) market participants.
Alternatively please contact us here box.futureofbalancingservices@neso.energy
| Name | Published |
|---|---|
| NESO Operational Metering Overview | 26 Jun 2025 |
| OBP Service Provider - Web Services Specification | 19 Dec 2024 |
| Slow and Quick Reserve - webinar slides - 9 March 23 | 16 Mar 2023 |
| Reserve Reform Show & Listen 2 - 26.05.22 | 27 May 2022 |
| Reserve Reform Show & Listen 1 - 28.04.2022 | 29 Apr 2022 |
| Slow Reserve Service Summary - April 22 | 21 Apr 2022 |
| Slow Reserve Feedback Proforma | 14 Apr 2022 |
| Reserve Reform Update - February 2022 | 25 Feb 2022 |
| Reserve Product Reform Consultation | 5 Mar 2021 |
Following our Request for Input on the proposed new Slow Reserve service, find out what's next for the service during our webinar on 11 February at 2pm. We'll deep-dive into the service and procurement design incorporating both Balancing Mechanism and non-Balancing Mechanism market participants.
19 May 2023
ESO has come to the decision to delay the delivery of the new Reserve reform products, Slow and Quick Reserve – originally planned for October and November 2023. As a result, we will not be launching our EBR Article 18 consultation to industry as planned at the end of May.
This decision has been taken in light of the significant changes that would have been required in our existing, legacy balancing systems and processes, given the complexity of the new service designs. In the midst of a complex and rapidly evolving systems change environment, we believe it is more prudent to re-evaluate these changes to consider if implementation into our legacy systems is still appropriate, as opposed to direct implementation into our Open Balancing Platform (OBP).
Postponing the rollout of our new Reserve services grants us the opportunity to re-examine our proposed service designs, evaluate our IT options, and collaborate with you more effectively. This will ensure that the best solutions are delivered and that the necessary updates to our balancing systems are apt for enhancing our operational toolkit and are better aligned with the implementation of our future systems.
In the coming weeks we will be looking to understand the extent to which we review the proposed services designs and IT solutions. We will then communicate how and when we intend to engage with you in this process via the Future of Balancing Services distribution list.
20 April 2023
Last month, we held a webinar on 9 March where we updated industry on our delivery plan and provided a refresh of the services (please refer to the Document Library tab for the documentation and recordings).
This delivery plan indicated launching our EBR consultation towards the end of April with the dependencies being further exploration of system changes and the quantum of change acceptable to market providers as well as the ENCC. We have also noted the feedback on Crossovers from the market and we are still analysing the most optimal solution for the end consumer.
As we are still working through these dependencies and feedback, the EBR consultation will be moved provisionally to the end of May.
We intend to update and confirm our delivery plan for Quick Reserve and Slow Reserve in Future of Balancing Services newsletter.
Slow Reserve Technical Onboarding Webinar 6 November 2025
Following the webinar held on 6 November 2025, we have shared the slides and a recording of the webinar.
Slow Reserve Webinar 11 February 2025
Following the webinar held on 11 February 2025, we have shared the slides and a recording of the webinar.
Reserve Reform Update Webinar 19 December 2023
Following the webinar held on 19 December 2023, we have shared the slides and a recording of the webinar.
Slow Reserve Consultations
Consultation Closed: 9th October 2025
On 9th October 2025, Ofgem set out its decision to approve the proposed Service Terms and Procurement Rules for the new Slow Reserve service.
As part of these proposed new Slow Reserve terms and conditions, NESO extend its product co-optimisation across the current day ahead auction platform so as to include Slow Reserve alongside Quick Reserve, Dynamic Response, and Balancing Reserve, resulting in a single, simultaneous, co-optimised auction.
On 1st August, NESO submitted to Ofgem the following documentation relating to proposed Slow Reserve Service Terms and Procurement Rules for it's consideration and expected decision on 1st October. Having consulted with industry, NESO also publishes its responses to feedback below.
Further enquiries
Get in touch with the right team within Balancing Services, who are on hand to answer any questions you may have.