How to complain about a back bill from your energy supplier

The protection that matters most is the 12-month rule. Ofgem's back billing rules mean a household does not have to pay for energy used more than 12 months ago where it has not had an accurate bill, statement of account, or a Direct Debit set too low¹. Which? states the same limit in plain terms: suppliers cannot back-bill for energy used more than 12 months ago, as long as they were at fault².

The route to a resolution runs through the supplier first, then the Energy Ombudsman. A complaint can go to the Ombudsman if a reported problem is not fixed within 8 weeks, if the household and the company cannot agree how to fix it, if a deadlock letter is received, or if the household is not happy with the decision³. Free advice sits alongside that process, and in Northern Ireland the advice body is different.

What a back bill is, and when it can be challenged

A back bill is not the same as an ordinary monthly or quarterly bill. It is a correction: the supplier has undercharged, usually because it billed on estimates, missed a meter exchange, or failed to bill at all. Confused.com describes energy back bills as a catch-up bill sent when a household has been incorrectly charged, covering extra energy used but not charged for over the past 12 months². Ofgem's own wording is narrower and more precise: the supplier asks for payment when it has not accurately charged for energy already used¹.

The right to challenge follows from that. A household can challenge a back bill if it believes the supplier has charged for energy used more than 12 months ago, by writing to the supplier and making a formal complaint if needed². The challenge is not a refusal to pay anything; it is a dispute about the period covered.

The rule has legal weight behind it. Standard Licence Condition 21BA requires consumer supply contracts to reflect the Backbilling Requirement, and any contract that does not is unlawful, with incompatible terms unenforceable⁸. That matters because it means the 12-month limit is not a goodwill gesture a supplier can set aside.

There are two important limits. First, the protection applies where the supplier is at fault. A household that has acted unreasonably, for example by stopping the supplier from billing accurately, including by blocking access to the meter, will have to pay for energy used more than 12 months ago⁴. Second, the rule is about domestic supply contracts. Where charges are recovered through a service charge, for example on some heat networks, the 12-month back billing limit does not apply, and a landlord has 18 months within which to notify a consumer of service charge costs under Section 20B of the Landlord and Tenant Act 1985 in England and Wales⁹.

The 12-month rule: disputing charges over a year old

The 12-month rule is the centre of any back bill complaint, and it is worth stating exactly. Ofgem's back billing rules mean a household does not have to pay for energy used more than 12 months ago if it has not had an accurate bill, statement of account, or a Direct Debit set too low¹. The clock runs from the point the error was detected, not from the date the bill was issued.

In practice, that means the first task is to establish when the energy was used and when the supplier found out. A bill covering 18 months of estimated readings will usually split into a protected period and a payable one. The protected part is the energy used more than 12 months before detection; the rest remains due.

The exception is the one most likely to be raised against a household. Where the household has acted unreasonably, for example by stopping the supplier from billing accurately, including by blocking access to the meter, the 12-month protection falls away⁴. Smart meter installations, refused appointments and unanswered letters can all feature in that argument, so the record of what was offered and when is worth keeping.

Two further points shape the outcome. The rule sits in the supply licence rather than in general consumer law, so it applies to domestic energy supply contracts and not to every charge that appears on a statement⁸. And where a property is on a heat network with charges bundled into a service charge, the 12-month limit does not apply at all; the relevant period is 18 months for notifying service charge costs in England and Wales⁹. Households in that position are disputing a different rulebook.

For a household's energy independence, the back billing rule is a rare thing: a statutory-style protection that limits how far a supplier can reach back into the past. It does not remove dependence on the supplier or the grid, and it does not settle the bill, but it caps the exposure.

How to complain to your supplier, step by step

The complaint starts with the supplier, and it starts in writing where possible. The Energy Ombudsman's guidance is direct: the first thing to do is log a dispute with the supplier¹⁰. Age UK's advice is to contact the customer services department to try to resolve it, using the contact details on the back of the bill or on the company's website¹¹.

A structured complaint is easier to assess. Smart Energy GB advises telling the supplier what the complaint is about, when the problem happened, what outcome is wanted, the first date the supplier was contacted about it, and details of letters, emails and phone calls¹². That list doubles as a checklist for the household's own file.

1. Contact the supplier's customer services department and log the dispute, using the details on the bill or website¹¹.
2. Set out what is disputed, the dates of the energy use, and what outcome is sought¹².
3. Keep copies of letters, emails and notes of phone calls, including dates¹².
4. Ask for the complaint to be treated as a formal complaint and note the date it was raised¹⁰.
5. If the response is unsatisfactory or none comes, ask for a deadlock letter or wait out the 8-week period³.

Where the property has its own contract with an energy supplier, the supplier's complaints procedure applies and its details should be on the bill or the website¹³. Where the property is on a heat network, the route differs, and the first step is to speak directly to the assessor or operator before going further¹⁴.

Getting free help from Citizens Advice

Citizens Advice is the main free route in England and Wales. Its consumer service can help with problems with energy bills or supply, and it is free⁶. It also offers free and impartial advice on making a complaint to an energy company or the Energy Ombudsman, by phone or online chat¹¹.

The service does more than explain the rules. It may refer a household to the Extra Help Unit where support is needed with a difficult or urgent complaint, where the household cannot deal with the supplier alone because of personal circumstances, where it is considered vulnerable, or where there is a risk of disconnection from an energy supply⁶. That referral is the point at which advice becomes active casework.

Citizens Advice also covers the wider problem. It offers information and support on struggling to pay bills, problems with an energy supplier or supply, saving energy at home and getting a better energy deal¹⁵. A household that is disputing a bill and worried about the next one can raise both in the same conversation.

Coverage is not uniform across the UK. Citizens Advice's consumer service covers England and Wales⁶. In Northern Ireland, the Consumer Council NI and Northern Ireland Energy Advice give free, independent and impartial energy advice to all domestic householders⁷. Scotland has its own advice landscape, and households there should check which body covers their case before starting.

Other free services exist alongside Citizens Advice. The Centre for Sustainable Energy offers free advice by message¹⁶, and National Energy Action runs a Benefits Advice and Claim Support Service that is free, confidential, independent, accessible and focused on supporting vulnerable households¹⁷. There is also a link into local authority support: a household referred to the local authority by its energy supplier or Citizens Advice because it is struggling to pay electricity or gas bills can qualify under ECO4 Flexible Eligibility¹⁸.

Taking your complaint further: the Energy Ombudsman

If the supplier does not resolve the complaint, the Energy Ombudsman is the next step. Ofgem's guidance is that a household can contact the Ombudsman if it is not happy with the way the supplier has handled the complaint¹⁹. The same escalation route applies to complaints about prepayment meter installations²⁰.

The Ombudsman is approved by Ofgem to deal with consumer complaints about energy bills, mis-selling, problems with energy supply and problems with switching supplier³. Its powers go beyond mediation. It can tell the supplier to look at the complaint again, look at the complaint and say whether the decision is right, make the supplier give a decision, and tell the supplier to pay financial compensation if it thinks that is right¹².

Eligibility is defined. A household can complain if a reported problem is not fixed within 8 weeks, if it and the energy company cannot agree how to fix it, if a deadlock letter is received, or if it is not happy with the decision received³. The Ombudsman can be reached on 0330 440 1624, with option 3 for heat networks when prompted⁵.

Evidence requirements are set out in the Ombudsman's own dispute forms. A complainant must have complained to the supplier already, match the company name on the account holder's bill, and have sufficient evidence including the date the complaint was raised²³. For final bill complaints, suppliers are expected to provide a copy of the final bill issued along with evidence of the date it was issued²⁵.

If the bill is disputed and unaffordable at the same time

Disputing a bill does not pause the account, so affordability and accuracy usually need handling together. Ofgem's guidance is that a household struggling to pay for energy, or expecting to struggle, can ask its supplier to agree a payment plan, a payment break or a reduction, to review payments and debt repayments, and to give access to hardship funds²⁶.

In Northern Ireland the same principle applies with different bodies. Suppliers can reassess or cut debt repayment and bill payments for domestic customers in financial trouble, and can refer customers who are struggling to pay bills to organisations such as Advice NI, which offers free, independent debt advice²⁷. Northern Ireland also has its own low carbon heating support, which is separate from the Great Britain schemes²⁸.

There is a wider context. Ofgem figures show combined domestic energy debt and arrears rose 5% from £4.55bn to £4.79bn between Q4 2025 and Q1 2026, up 15% on Q1 2025²⁹. A back bill arriving on top of existing arrears is therefore a common situation rather than an unusual one.

Two practical points help. First, a credit balance on an account can be claimed back at any time, and compensation may be available if the supplier does not refund it³⁰. Second, where a supplier has failed or is in administration, the ordinary complaint and warranty expectations change, and Ofgem publishes what happens when a supplier goes out of business³¹. Households in that position should treat the supplier's own promises with caution and follow the official process.

For energy independence, the honest position is that a back bill dispute is a consumer protection, not a route to self-sufficiency. It limits how far a supplier can reach back and gives a household a free escalation route, but the account, the meter and the supply remain the supplier's. The protection is real and worth using; it does not remove the dependence.