Slow reserve

Reserve is needed for frequency management when there is an imbalance between supply of energy and demand for energy.

System conditions are changing, and faster-acting services procured closer to real-time are required to meet three distinct criteria: a) to restore frequency to within statutory limits within 60 seconds b) to recover frequency to within operational limits within 15 minutes and c) to respond to transient supply demand imbalances that take pre-fault frequency close to operational limits

A suite of new Reserve products are being designed to replace the existing suite of positive and negative Reserve products: Short Term Operating Reserve (STOR), Fast Reserve (FR) and Optional Downward Flexibility Management (OFDM).

Slow Reserve, separated into Negative Slow Reserve (NSR) and Positive Slow Reserve (PSR), will be the first of our series of new Reserve products. It is designed to operate post-fault and aims to provide ESO with access to firm, bi-directional energy to displace large losses on the system and recover frequency to ± 0.2Hz within 15 minutes​.

Further enquiries

[email protected]

Latest News and Updates

26 May 2022

We are pleased to share a summary of our proposed product and service design for two new Slow Reserve products - Positive Slow Reserve and Negative Slow Reserve. There are still some areas of the services which we would like to seek your input on and will do so through our series of Show & Listen events. Please use following links to check the slides of Show & Listen 1, Show & Listen 2, and Reserve Reform Q&A.


4 March 2021

We are co-creating the new reserve product suite with the industry, and this consultation aims to gather industry views on initial product designs which have been based on feedback from the industry workshop on 9 December 2020.  Please check the Design Consultation Document and Slow Reserve Proforma for more details.


25 February 2022

We shared an update on Reserve Reform project delivery. We will not be launching Negative Slow Reserve directly after Dynamic Moderation in Summer 2022, and instead we will be taking the time to further engage with industry to identify the optimum approach to deliver the new reserve products. More information can be found in our Project Delivery Updates.

How to participate

More information will be provided shortly

Key documents and guidance

Below you can find the documents relating to Slow Reserve.

The other service in Reserve Reform

Team working at desk from above with glow lines

Quick Reserve

Quick Reserve, separated into Negative Quick Reserve (NQR) and Positive Quick Reserve (PQR), is aimed primarily for reacting to pre-fault disturbances to restore the energy imbalance quickly and return the frequency close to 50.0 Hz.

Quick Reserve