An Open Letter on Energy Supplier Invoicing Failures

Energy prices are under renewed pressure: drought and wildfires straining supply, a war in the Middle East driving up the cost of raw materials, squeezing the purchasing power of households and the investment capacity of businesses alike. The government’s decision to scrap the 5% VAT on electricity bills from 1 October 2026 only underlines how urgent that pressure has become, and it does nothing for businesses, who will still face higher energy costs with less room to invest, hire, or stay competitive.

Optima Energy is an energy data management platform that has helped businesses and institutions validate their energy bills for over 30 years, dissecting complex, multi-site invoices to catch billing errors, deliver cost control, and surface insights buried in the data, ensuring customers pay only for the energy they’ve actually consumed, while supporting their Net Zero reporting obligations. We are writing this letter because inaccurate billing is, and has been for over a decade, the single largest complaint from UK energy customers. It isn’t an occasional failure, and it isn’t a new problem overtaking an otherwise well-run industry: it is the predictable result of suppliers treating invoicing accuracy as optional for far too long, operating under the strain of tech debt and outdated systems, with under-trained teams, and without the investment needed to fix their invoicing practices.

We and our co-signatories sign this letter as third-party intermediaries and market participants for whom, in Optima’s case, fixing these invoicing failures is the reason we exist. In this economic climate, every pound our partner businesses lose to an inaccurate, late or unresolved energy bill is a pound that cannot be spent growing, hiring staff, or sometimes staying open.

Let’s look at some evidence. Billing was the largest single category of complaint received by the Energy Ombudsman in every year it has published data, 56% of all cases in 2025 and 58% in 2024. Citizens Advice helped someone with an energy billing problem roughly every two minutes from January to October 2024, an 83% increase on pre-energy-crisis 2020, with nearly a quarter of those cases involving a ‘shock’ catch-up bill now averaging over £2,500, up from under £1,700 just two years earlier. Citizens Advice’s research into small and micro businesses found trust in the market so eroded that many business owners no longer bother raising billing issues at all, meaning the official complaint figures likely understate the true scale of the problem.

More frequent back-billing is the clearest example of an industry that has not fixed the underlying issues, even when they’ve been repeatedly told exactly what to fix. All the way back to 2007, Ofgem set out its back-billing principle: no charging customers for supplier-caused errors older than 12 months. Ofgem were still flagging inconsistent supplier compliance in 2011, still launching a fresh project to address it in 2017, and still having to move the principle from voluntary code to statutory licence condition later that year because self-regulation hadn’t worked. Today, the Energy Ombudsman’s own consumer guidance acknowledges that ‘many people report problems with recalculated bills that go back further than 12 months’, and it handled 3,218 separate back-billing disputes in 2024 alone, nearly two decades after suppliers first agreed this shouldn’t happen.

While the above statistic applies to domestic energy customers, business customers fare no better. Ofgem’s research found billing was the most common reason non-domestic customers complained in 2024: cited by 52% of complainants, up from 23% the year before and that 47% of those complaints went unresolved by the supplier itself. As one MP recounted in Parliament last year, a constituent who had run her high-street café for more than 35 years was sent a £10,000 demand with seven days to pay and a threat to disconnect her electricity as a result of her supplier confusing her day and night meter readings. A review of her account going back to 2017 found the supplier owed her money, not the other way around. That case reached Parliament. Most don’t; they land instead on the desks of TPIs and brokers, who are left to fight suppliers’ own errors on their customers’ behalf.

Underneath all of this sits a quieter failure: when a complaint isn’t resolved, suppliers correctly signpost customers to the Energy Ombudsman only 48% of the time, a basic regulatory obligation they have failed to meet consistently for years. Billing and metering data quality fares no better, with Ofgem’s own 2023 review rating none of the 17 largest suppliers it assessed as having ‘no significant issues’. These are not edge cases…

Just this summer, in an eight-week sample, Optima validated 353,067 business energy bills; a third of them, with a combined value of £429 million¹, failed at least one accuracy check.

And it is about to get even more complex for energy suppliers. Half-hourly settlement does not just multiply the number of meter reads, from a single estimate to 48 real readings a day; it is unlocking a wave of flexible, time-of-use and index-linked pricing contracts that suppliers must now price and reconcile per settlement period, not per bill. Full migration is mandated by May 2027, with restrictions on registering new customers for suppliers who miss it. An industry that cannot yet reliably issue an accurate monthly bill is about to be handed a billing computation problem, with nothing in its record on invoicing accuracy to suggest its systems, meter-read handling, or under-trained teams are ready for it. More than a quarter of the business bills Optima processes today are billed on estimated rather than actual meter reads. An industry still estimating a quarter of its monthly bills is not ready to reconcile forty-eight settlement periods a day.

Optima processes this data daily. In the first eight months of 2026 alone², we independently recalculated £1.8 billion of business energy bills and identified £167 million³ in discrepancies⁴, errors in both directions, with two in three supplier billing files containing at least one. Furthermore, in the last fourteen months alone, 10,841 supplier billing files reached us in a state that required manual intervention before the bill could even be checked. Across the 2.7 million invoices we have processed since January 2024, 56,011 of them, worth £246 million, were issued more than twelve months after the end of the period they covered. The problem is not evenly spread across the 40 suppliers we measure: the median issues 1.11% of bills more than a year late, but 12 of them exceed 2% and the worst issues roughly one bill in fourteen more than twelve months late. Good practice is clearly achievable. These are not figures drawn from complaints, surveys or self-reporting. They are what arrives in Optima’s systems from suppliers, every day, at scale.

In conclusion, the cost falls hardest on the businesses least able to absorb it. Disputed and back-dated bills are a direct cash-flow threat, and in the worst cases (like the café owner above, or the countless smaller cases that never reach Parliament) a threat to the business’s survival and the jobs that depend on it. For TPIs and brokers, every hour spent chasing a supplier’s error is an hour not spent delivering the cost control, net zero reductions and market insight their customers actually rely on them for. It is, ultimately, a cost the industry imposes on itself too: Citizens Advice estimated suppliers were spending £125 million a year handling billing complaints as far back as 2015, and there is no evidence that figure has meaningfully improved since.

What we ask

To the energy suppliers

1. Commit to independent assurance of billing accuracy, measured against a common published standard, with results reported publicly each year.
2. Work with Ofgem and consumer bodies to develop a common invoice-accuracy standard and a public SLA for resolving billing queries, including those raised by TPIs and brokers on a customer’s behalf.
3. Treat meter-read handling and billing data quality, including the data fed to broker and TPI systems, as a measurable service commitment, and confirm that billing systems and processes are ready for market-wide half-hourly settlement well ahead of the May 2027 migration deadline.
4. In the short-term, use the technology already available such as EDI (Electronic Data Interchange) files, to give every customer structured, machine-readable invoice data, not just PDF bills.⁵
5. Apply the 12-month back-billing principle consistently and without exception.
6. Apply the same complaint-resolution timescales and signposting obligations to billing disputes raised by TPIs and brokers on a customer’s behalf as to those raised directly by customers.
7. Correctly signpost every unresolved complaint to the Energy Ombudsman, as already required, and publish signposting compliance rates.

To government and Ofgem

1. Mandate a formal review of suppliers’ billing practices and require suppliers to demonstrate, through independent audit, that their invoices meet a published accuracy standard.
2. Extend the 12-month back-billing cap, written into supplier licence conditions, to all non-domestic customers, with meaningful penalties for breach.
3. Introduce mandatory compensation for customers and businesses when billing errors are supplier-caused, in line with standards already applied in other regulated utilities.
4. Independently verify and publish each supplier’s readiness for market-wide half-hourly settlement well ahead of the May 2027 deadline, and strictly enforce the restriction on registering new customers against any supplier that misses it.
5. Require suppliers to publish complaint-resolution and signposting performance at individual supplier level, reported separately for queries raised directly by customers and those raised via TPIs and brokers, so poor performers and delays pushed onto intermediaries cannot hide inside industry-wide averages.
6. Confirm and publish a firm timetable for the new TPI regulatory regime announced in October 2025.

This letter has been sent to Ofgem, the Department for Energy Security and Net Zero, and a number of specialist energy publications. It has also been submitted as late evidence to the House of Commons Energy Security and Net Zero Committee’s inquiry into business energy.

Optima Energy’s aim is to raise standards across the industry for the benefit of customers. As a first step, it will make its methodology and anonymised, aggregated data available to Ofgem, DESNZ and the Committee for independent scrutiny. We invite fellow industry participants (TPIs, brokers, software providers and energy suppliers) who share this view to add their names by signing the letter or contacting us at team@optima.energy.