MHHS FAQ – ENGIE UK

Market Wide Half Hourly Settlement (MHHS) is a major industry initiative to transform the way the electricity market operates. Under MHHS, all electricity customers will have their electricity consumption settled every half hour. Market-wide Half-Hourly Settlement (MHHS) is a key enabler of the flexibility to support the transition to Net Zero. It will encourage more flexible use of energy and help consumers lower their bills. MHHS will see a significant move forward in how electricity is used and accounted for and will introduce a faster, more accurate settlement process, introducing site specific reconciliation using half-hourly consumption data for all.

MHHS is being introduced because the current model for settling electricity consumption is not suited to the way in which we generate and use electricity today, nor how we expect to generate and use power in the future. The UK has a target to move to a zero-carbon electricity grid by 2035. In the Energy White Paper ‘Powering our Net Zero Future’ the government has set out an ambitious programme to build a fairer greener energy system. All this means that a larger proportion of our electricity is being generated by intermittent renewable sources of generation, such as wind or solar. At the same time, the demand on our electricity grid is increasing due to, amongst other things, greater adoption of electric vehicles and greater use of electricity for heating. MHHS will modernise this system, allowing suppliers and the grid to settle electricity consumption based on actual consumption data. It will contribute to a more cost-effective electricity system, encouraging more flexible use of energy and helping consumers lower their bills.

Settlement processes calculate how much energy has been consumed and generated, with charges levied on suppliers when energy used and purchased doesn’t balance. Under the current system, electricity consumption is accounted for and settled based on half-hourly (HH) and non-half hourly (NHH) consumption data which is collected by NHH and HH Data Collectors. MHHS will introduce half hourly consumption data for all electricity sites, increasing the accuracy of the settlement calculations and the overall efficiency of the market. MHHS will reduce the time taken to settle energy usage from 14 months to 4 months. The reduction in the settlement timeline is planned for July 2027.

The Market-wide Half-Hourly Settlement (MHHS) programme is progressing through a phased rollout between 2025 and 2027. Suppliers and meter agents are onboarded in qualification waves; they qualify once they have demonstrated, through formal testing and assurance, that they are able to operate within the new framework.

Key Dates:

The Market-wide Half-Hourly Settlement (MHHS) programme is progressing through a phased rollout, with industry testing currently underway. This testing ensures that suppliers and meter agents have the systems and processes in place to operate within the new MHHS framework. Here are some of the key milestones for the MHHS programme.

Date	Milestone	Details
September 2025	M8 / M10	MHHS changes officially went live on 22 September 2025, including: • Industry code changes • Central services and systems going live • Updates to the format of the Supply Number • Support for migration to and operations under new MHHS arrangements for MHHS qualified suppliers • Support for and reverse migration to pre-MHHS arrangements for suppliers that are not MHHS qualified
October 2025	M11	The migration window opened for suppliers and agents who have qualified. Migrations will be phased across 2025, 2026, and 2027 depending on when each supplier qualifies.
May 2027	M15	All MPANs, suppliers and their agents must be fully transitioned to operate under MHHS arrangements.
July 2027	M16	The new, faster settlement timetable goes live. The MHHS operating model is fully in effect across the entire market.

ENGIE electricity consumers do not have to do anything right now to move to MHHS, this transition will be managed by ENGIE. A small number of customers may need to agree a site supply capacity with their Distribution Network Operator after the transition. For these customers we will be in contact directly to explain what they need to do.

No, the programme is designed to be a phased rollout so suppliers will qualify and migrate customers at different times within the overall MHHS programme timings.

If you have a customer who is currently with a supplier that has migrated and move to ENGIE, there are clear industry-wide processes to ensure that there is no impact to customers. Any customers already migrated would be ‘reverse migrated’ and for those moving from a non-qualified supplier to a qualified supplier they would be ‘forward migrated’ Both these processes will happen automatically as part of all suppliers’ system readiness for MHHS. This means suppliers including ENGIE, until they qualify, will operate in a dual environment where both legacy and MHHS processes will run side by side from October 2025.

There will be no change to your customers’ billing. The only difference they will see on their bill is the change to the Supply Box Number Format which can be seen below, often referred to as the S box.

No there is no real change for your customers right now except for the Supplier Box Number format changes. If your customer pays via EDI, we are contacting them directly as the metering switch code number will be replaced by the standard settlement configuration number, as shown above. The number will move from a 3-digit number to a 4-digit number. The line loss factor number will also change name to DUoS tariff ID once a customer is migrated to MHHS, but the number will remain the same. In the short term until migration little will change, and your customers will continue to receive and pay their invoices as normal.

ENGIE is actively preparing for the Market-wide Half-Hourly Settlement (MHHS) programme and is progressing through the necessary steps to ensure a smooth transition. We’re committed to keeping you informed and will provide advance notice before any migration activity begins in 2026, ensuring you have time to understand what it means for you.

For now, there are no changes in the way you or we will provide quotes for customers. As we move through our ENGIE MHHS timeline we will continue to update you on any changes that may impact this.