Bill validation under Market-wide Half-Hourly Settlement still means checking an invoice against the contract and the evidence behind it. For sites in Great Britain with time-of-use or interval-based charges, that includes checking the relevant half-hourly periods. Migration alone does not change a flat-rate retail agreement. Milestone 14 on 28 October 2026 is the supplier-readiness deadline, not a single switch-over date for every client's bill.
Checking the unit rate, standing charge and total consumption remains useful. The extra work is knowing which data is actual, which is estimated and which charging components depend on time, capacity or a fixed site band. Those distinctions matter more than assuming every non-commodity charge moves with each half hour.
This guide covers the validation risks to review during migration, the evidence needed to investigate them and a practical checklist for British electricity portfolios.
What changes about bill validation under MHHS?
The contract determines the check. Reconcile what was charged against agreed rates and metered or estimated volumes. Where an applicable charge varies by time, use the interval series and the correct time windows. Fixed and bundled charges still need their own checks rather than being forced into a half-hourly calculation.
The new operating model uses market segments and data services rather than the old half-hourly/non-half-hourly split. Actual readings are used where available; load shaping and estimation remain necessary where they are not. A smart meter is not proof that every interval is measured. Identify the source, quality flags and revisions in the series you receive before using it to validate a bill.
The third change is the one brokers underestimate. Half-hourly data raises precision, and accuracy still has to be checked separately. An error expressed to more decimal places is still an error, and the same granularity that makes settlement fairer gives a mistake far more places to hide.
Which errors should brokers check during migration?
These six risks are worth checking during migration. They can occur under existing arrangements too, and a plausible monthly total does not rule them out.
Time-band misallocation. Where the applicable Distribution Use of System (DUoS) tariff has time bands, reconcile consumption to the network operator's published windows and rates. Bands vary by network, tariff and charging year, so do not assume one universal red-band window. Confirm how those network costs are recovered under the customer's supply contract.
Estimated intervals treated as measured. Gaps may be filled with substituted or estimated values. Check the series' quality indicators and ask for supporting data where the invoice does not make the distinction clear. More decimal places do not make an estimate an actual reading.
Restated volumes arriving as a reconciliation line. Late data can revise settlement volumes and lead to billing adjustments. Elexon's transition design plans a reduction in final reconciliation from 14 months to seven months from 1 April 2027, then four months from 2 July 2027, subject to programme decision points. Trace any supplier adjustment to its periods and contract basis; the market settlement timetable is not itself the customer's billing or refund deadline.
Non-commodity charges applied on the wrong basis. NESO's final 2026/27 TNUoS tariffs include a non-locational demand residual charged per site per day by band, separately from locational demand charges. For example, Table 10 gives the LV2 residual tariff as £11.511840 per site per day for 2026/27. That is one component for one band, not a total bill or a rate for every low-voltage site. Check the assigned band, charging year and contractual pass-through basis rather than applying a market-average increase.
Levies not reconciled with taxable volume. The main Climate Change Levy rate for electricity and gas is 0.801p per kWh from 1 April 2026. Check eligibility for exemptions or reduced rates, and reconcile any volume correction to the levy treatment that applies. A change in consumption does not justify applying the full rate to an exempt supply.
Overlapping agent charges after a switch. Check meter and data-service agreements, termination dates and invoices together. Where charges overlap, establish whether they cover different services or represent a duplicate before raising a claim.
How long do you have to find an error?
Ofgem's 12-month back-billing protection covers domestic and qualifying microbusiness customers in specified circumstances, including where they have not been correctly billed. Exceptions apply. Check the client's eligibility and the reason for the bill rather than extending the rule to every SME or larger business.
Back-billing protection and recovery of an overpayment are different questions. Keep the contract, invoice history and supporting data for any refund query. Do not treat the 12-month back-billing rule or the market settlement timetable as a universal deadline for recovering an overcharge.
The practical rule is to investigate exceptions within the billing cycle while records and contacts are available. Regular validation makes corrections easier to explain than a retrospective sweep at renewal.
What does a post-MHHS validation check look like?
Use these eight checks as a repeatable review sequence, starting with data quality and the contract.
- Check the data quality flags first. Establish which periods are measured, which are estimated, and which are shaped. Everything downstream inherits the answer.
- Reconcile at the level each charge requires. Use interval calculations for time-dependent components and billing-period checks for fixed or bundled charges. Reconcile both the supporting detail and the total.
- Check applicable time bands separately. Use the actual DUoS windows and any time-of-use periods in the supply contract, including the correct time-zone and daylight-saving treatment.
- Trace every reconciliation line to its periods. A net adjustment with no period breakdown is a query until the supplier shows the periods. Ask for the breakdown as a matter of routine and suppliers start providing it.
- Separate distribution, transmission and levies. DUoS follows the relevant distribution charging statement; TNUoS follows NESO's transmission tariffs. Check whether the supplier contract fixes, bundles or passes through each component, using the rates for the period billed.
- Hold a rate card per meter point. Contracted rates, effective dates, and which elements are fixed against which are passed through. Most contract mis-application errors are an old rate surviving a renewal date, and they are only visible if the correct rate is recorded somewhere machine-readable.
- Track agent charges across switches. Keep meter operator and data service arrangements on the same register as the supply contract so a duplicate is visible the month it starts.
- Store the evidence with the query. Include the relevant data, rate version and recalculation so the supplier can investigate the difference. Evidence supports a claim; it does not guarantee a refund.
A 30-day period contains 1,440 half-hourly intervals per meter. The workload comes from managing those records alongside rate versions, data-quality flags and exceptions across the book. A controlled process helps preserve coverage and a clear explanation of each adjustment.
How Utilified approaches validation at half-hourly granularity
UMS supports utility operations and advisory work alongside your CRM, bringing accounts, sites, meter records, invoices, contracts and tariffs together. Supported intake, configured comparisons and exception review help teams keep the evidence for a British portfolio in one workflow. Feed coverage, validation depth and tariff support must be confirmed for the supplier and service being delivered.
Invoice intake and extraction support structured records and review against source documents. Joule is an add-on for assisted questions and workflows where enabled. Confirm the available parser, data route, permissions and review process rather than assuming every invoice or charge can be handled automatically.
The Custom Portal add-on provides configured branded client views, with reporting controlled by account access and setup. It can support delivery of your utility service; the data available to clients and the outcomes you can promise depend on coverage, configuration and the work completed.
Explore the Utilified platform →
Frequently asked questions
What is bill validation under MHHS?
It is checking the invoice against agreed charging rules and supporting data. Use half-hourly calculations where time-dependent charges apply, alongside checks of fixed charges, totals, billing dates and pass-through items.
Does MHHS make energy bills more accurate?
More granular settlement can improve the evidence available, but it does not guarantee an accurate retail bill. Check estimates, timestamps, revisions and contract application separately.
How far back can a supplier back-bill a business in Great Britain?
Ofgem's 12-month protection applies to qualifying microbusiness customers in specified circumstances, with exceptions. Larger businesses should not assume the same protection. Check the customer's eligibility and the facts of the bill.
What data do you need to validate a half-hourly bill?
Four things: the half-hourly consumption series with its data quality flags, the contracted rate structure for that meter point including effective dates, the applicable published charging statements for network and levy charges, and the invoice broken down to line-item level. Holding only the invoice lets you check the arithmetic and little else.
When do the MHHS changes take effect?
Migration is underway. Supplier readiness is due on 28 October 2026, migration completion is targeted for 7 May 2027, and the shorter settlement timetable is planned for 2 July 2027. The transition design includes an interim seven-month final reconciliation period from April 2027.
Related reading:
- What MHHS means for energy brokers in 2026
- What is invoice validation in energy management?
- Manual vs automated invoice validation
- What is interval meter data? A complete guide
- Utility management software vs spreadsheets: why growing portfolios need a platform
References
[1] MHHS Programme: Key Programme Milestones. M14, M15 and M16 targets.
[2] Elexon: MHHS Service Definition Document. Load shaping, estimation and data services.
[5] Ofgem: Help with business energy bills, including microbusiness back-billing protection.