Hit With a Shock Energy Bill? The Back-Billing Rule That Can Wipe Out Charges Over 12 Months Old (UK)

You open a bill from your gas or electricity supplier and the figure does not make sense — a "catch-up" charge for hundreds or even thousands of pounds, for energy you supposedly used months or years ago. Perhaps the meter was never read, an estimate was wildly wrong, a switch went haywire, or a smart meter quietly stopped sending readings. It feels like a bill you have no choice but to pay. Very often, it is not. UK energy rules contain a powerful protection against exactly this, and most people have never heard of it. Here is how the back-billing rule works, and how to challenge an unfair bill in writing.

The 12-Month Back-Billing Rule

Your energy supplier is regulated by Ofgem, and its licence to operate contains a binding condition on back-billing. In plain terms: a supplier cannot charge you for gas or electricity you used more than 12 months ago if it failed to bill you correctly for that energy at the time — and the failure was not your fault. This covers both your unit charges and standing charges. If the supplier could have billed you accurately but did not, the charges older than twelve months are, in effect, off the table.

This is not a discretionary goodwill gesture. It has been a binding rule for domestic customers since 1 May 2018 (before that it was only a voluntary industry code that not every supplier honoured), and the same protection was extended to microbusinesses from November 2018. It applies regardless of how you pay — a credit (billed) account or a prepayment meter account are both covered when a supplier tries to recover backdated charges over twelve months old.

What Counts as the Supplier's Fault

The rule is aimed at billing failures the supplier could and should have avoided. Typical situations where back-billing protection applies:

• The supplier estimated your bills for a long period and never chased an accurate reading, then landed you with a large correction.
• A smart meter stopped sending readings and the supplier failed to notice or act.
• The supplier had your readings but did not bill them correctly, or lost track of the account after a switch or a house move.
• A meter mix-up meant you were billed on the wrong meter, or not billed at all.

In all of these, the common thread is that the supplier had the opportunity to bill you properly and did not. That is what triggers the protection.

When the Protection Can Fall Away

It is important to be straight about the limits, because a supplier will look for a reason the rule does not apply. The protection can be lost where you — not the supplier — are the reason the account could not be billed accurately. Ofgem's stated examples of this centre on obstruction: blocking or refusing access for meter readings, tampering with the meter, ignoring the supplier's reasonable requests for payment or account information, or theft of energy. What does not, on Ofgem's own guidance, automatically strip your protection is simply having suspected your bills looked low and not flagging it — the test is about active obstruction, not passive silence. If a supplier claims you have "forfeited" the rule, ask them to point to the specific conduct they say caused the billing failure.

A Note on Timing — the Rule Is Under Review

As of mid-2026 the back-billing limit is 12 months, and that is the figure to rely on today. Be aware, though, that Ofgem has been actively reviewing the rule after a wave of upheld back-billing complaints, and has floated shortening the window — potentially to six months, particularly for smart-meter customers whose readings should be accurate in near real time. No change to the twelve-month figure has been made law at the time of writing, but if you are reading this some time after publication, it is worth a quick check that the period has not been tightened further in your favour.

Before You Write: Gather Your Evidence

• Find the bills. Work out exactly which period the catch-up charge covers, and identify which portion of it relates to energy used more than 12 months before the date of this new bill.
• Dig out your meter readings — any readings you submitted, photos of the meter, or smart-meter data — and note any long runs of "estimated" bills.
• Keep the correspondence — anything showing you gave access, submitted readings, or told the supplier something was wrong.
• Do the split. You are not necessarily disputing the whole bill — only the part that falls outside the twelve-month window. Being precise about that figure makes your challenge much harder to brush off.

What Your Letter Should Say

A clear, factual letter that names the rule tends to get a very different response from a phone call, because it shows the supplier you know the protection exists. Your letter should:

• Identify the account — your account number, the supply address, and the meter(s) concerned.
• Set out the bill you are challenging — the amount, the date it was issued, and the period it claims to cover.
• Invoke the back-billing rule — state that Ofgem's back-billing protection prevents the supplier from charging for energy used more than 12 months before the bill was issued, where the supplier failed to bill accurately and you did not obstruct that.
• Identify the disputed portion — the specific amount that relates to consumption older than twelve months, which you are asking to be removed.
• Ask for a corrected bill and, if you have already paid, a refund of the amount wrongly charged.
• Set a clear deadline — commonly 14 days — and state what you will do next if it is not resolved.

If the Supplier Refuses

If the supplier rejects your challenge or does not resolve it, you can escalate. Raise a formal complaint through the supplier's complaints process first. If you have not reached a resolution after eight weeks, or the supplier sends you a "deadlock" letter sooner, you can take the case to the Energy Ombudsman — a free, independent service for domestic and microbusiness customers. If the Ombudsman finds in your favour and you accept its decision, the supplier is expected to put things right (for example, correct the bill, refund you, or pay any award). It is fair to note that the enforcement of Ombudsman decisions has been criticised and is itself the subject of reform proposals in 2026 — but for the great majority of cases, an Ombudsman decision in your favour is what settles the matter.

Throughout, keep paying for your ongoing, correctly billed usage so the dispute stays focused on the backdated charge alone.