DESNZ, supported by RECCo, has published the Government Response to the consultation on the TI Phase 1a Technical Specification, as part of the Smart Secure Electricity Systems (SSES) programme.
The publication follows a targeted industry consultation held between 27 August and 17 September 2026. Feedback from seven organisations has informed updates to the technical documents ahead of launch.
What is Tariff Interoperability?
Tariff Interoperability will provide a standard way for participating electricity Suppliers to share tariff pricing information through public application programming interfaces (APIs). This will allow Energy Smart Appliances (ESAs), such as smart electric vehicle chargers, heat pumps, and other services to respond automatically to electricity prices.
The aim is to help consumers make better use of cheaper electricity when demand is lower, while supporting a more flexible and efficient energy system.
What has changed following the consultation?
The government has confirmed that the TI Phase 1a technical documents will be incorporated into the REC, with a limited number of updates. These include:
• reducing data transfer size: The updated specification includes gzip compression for Get Tariffs and Get Tariff Details responses to reduce the amount of data sent when users request tariff information.
• making it easier to check for updates: Conditional HTTP requests (ETag and Last-Modified) have been added to allow users to check whether tariff information has changed without downloading the same data again.
• improving consistency: Inconsistencies between the TI API Technical Specification and the Energy Market Data Specification (EMDS) have been corrected, including data item names, definitions and payload examples.
• updating references: Temporary market message and data item references have been replaced with their permanent references.
• clarifying which tariffs are covered: Further guidance now explains tariff scope, meter-dependent time-of-use arrangements (such as Economy 7) and feed-in Tariff export payments.
• explaining how to identify meter-dependent tariffs: For Phase 1a, guidance explains how Suppliers can use the existing Tariff Comments field to identify where a tariff’s time-of-use periods depend on the meter configuration. Further design work on representing these arrangements will be considered through Phase 1b.
• providing temporary flexibility on response retries: The REC Performance Assurance Board (PAB) has granted a derogation from the TI Energy Supplier response retry requirements in REC Schedule 35, which will remain in place until the Schedule is formally updated.
These updates aim to give TI Energy Suppliers and TI users a clear and stable basis to complete build and testing ahead of launch.
Published documents
The Government Response is published alongside three technical annexes, which set out the detailed design for TI Phase 1a implementation.
- Tariff Interoperability: Technical Specification Government Response
- Annex B – TI API Technical Specification: a new REC Category 3 document, hosted on the REC Developer Portal
- Annex C – Energy Market Data Specification (EMDS): new and amended TI market messages and data items
- Annex D – Standards Definition Document: amendments to support the TI message type, market roles and YAML naming conventions
- FAQ
Next steps
The TI Phase 1a technical artefacts will be incorporated into the REC through the 6 November 2026 REC release. TI Energy Suppliers must build their systems against the published documents, complete the mandatory testing set out in REC Schedule 35, and confirm readiness through a director-level attestation by 8 January 2027.
TI users planning to use the public route are also encouraged to build and test their integrations against the published package.
For questions about implementation or testing, contact: tariffinteroperability@retailenergycode.co.uk
Key dates
- 29 October 2026: Government response and updated technical documents published
- 6 November 2026: REC release incorporating the TI phase 1a technical documents
- November 2026: TI Phase 1b design consultation
- 31 December 2026: Deadline for Suppliers to provide end points
- 8 January 2027: Deadline for TI Energy Supplier readiness attestations
- 18 February 2027: TI Phase 1a go-live
- November 2027: Planned TI phase 1b integration with the Consumer Consent Service (CCS) via private route
