Your Supplier Can't Bill You for Energy Used Over 12 Months Ago

A bill lands for £1,400 covering three years of energy nobody ever charged you for. It feels like a debt you have somehow accrued without noticing. In most cases it is not, and there is a rule that says so.

Standard Licence Condition 21BA is a condition of every domestic gas and electricity supply licence in Great Britain. It has applied to every licensed supplier since 1 May 2018, and it says a supplier must not charge you for energy used more than 12 months before the date of the bill, where the supplier is the reason you were not billed accurately in that time.

Microbusinesses get the same protection, from 1 November 2018.

The two words that decide every case

Almost every complaint that fails, fails on one of these.

"12 months before the date of the bill." The clock runs backwards from the date printed on the bill. Not from when the meter was last read. Not from when you noticed. Not from when you first complained. A bill dated 3 September 2026 cannot properly charge you for energy used before 3 September 2025.

"Where the supplier is the reason." The protection exists for supplier failure — a meter never read, a reading ignored, an account left unbilled, a switch that went wrong. It is not a general rule that old debt expires.

The distinction that catches people out

Before you complain, separate two things that look identical on a bill and are not.

Unbilled energy. The supplier never charged you for it at all. This is what 21BA covers.

A catch-up on estimates. The supplier did bill you, but on estimated readings that were too low, and is now correcting to an actual reading. This is usually not back-billing in the 21BA sense — you were billed, just inaccurately, and the correction can legitimately cover a longer period.

The same goes for a bill you simply did not pay. If the supplier issued an accurate bill at the time and you did not pay it, chasing you three years later is debt collection, not back-billing. 21BA does not write that off.

When the protection does not apply

The condition sets out its own exceptions. The one that comes up in practice is obstructive or manifestly unreasonable behaviour — refusing meter access, tampering with the meter, or refusing to engage at all.

Suppliers do sometimes reach for this. If yours does, the important point is that it has to evidence it, not simply assert it. Ask what specific conduct it is relying on and on what dates.

What gets written off

Everything attributable to the period beyond 12 months: the unit charges, the standing charges, and the VAT on both. It is not only the energy.

So the arithmetic is straightforward. Find the date of your first accurate bill. Count back 12 months. Everything before that line, where the supplier was at fault, should come off.

Are you a microbusiness — and is that even the right question?

If this is a business supply, the protection applies to microbusiness consumers, and that is a narrower category than it sounds.

A microbusiness is broadly one with fewer than 10 employees or full-time equivalents and annual turnover or balance sheet below €2m, or one using less than 100,000 kWh of electricity or 293,000 kWh of gas a year.

There is a trap here worth knowing about. Ofgem also recognises a wider Small Business category for certain other protections, and the two are not the same thing. A business can be small enough to use the Energy Ombudsman and still too large for the automatic back-billing protection. If a supplier or an ombudsman starts using "small business" and "microbusiness" as though they mean the same thing, ask which regulatory category is actually being applied to you.

How to challenge it

1. Get the breakdown first. Ask for the charge split by supply period, and the full estimated and actual reading history. You cannot argue a date you cannot see.

2. Identify your first accurate bill and the date on it. That is the anchor for everything.

3. Write to the supplier and name the condition. Do not just say the bill is old. Say that the charges for the period before [date] relate to energy used more than 12 months before the date of accurate billing, that the failure to bill was the supplier's, and that under Standard Licence Condition 21BA those charges cannot be recovered. Ask for a corrected bill.

4. Keep paying what you genuinely owe. Disputing the old portion does not excuse the recent portion, and paying the undisputed part keeps you out of collections while the argument runs.

5. Escalate at 8 weeks. If it is unresolved after eight weeks — or sooner if the supplier issues a deadlock letter — take it to the Energy Ombudsman. It is free for domestic and microbusiness customers, and its decisions are binding on the supplier but not on you.

If the ombudsman gets it wrong

This happens often enough to plan for. Ombudsman decisions sometimes lean heavily on the supplier's account of events. If that happens, point directly at the wording of the licence condition rather than arguing about fairness in general terms. The condition is not a courtesy the supplier extends — it is a binding obligation on its licence, and referencing it specifically changes the conversation.

If a supplier still refuses to comply with a licence condition, that is a matter you can raise with Ofgem.

If you cannot pay in the meantime

A large unexpected bill and a supplier refusing to budge is a genuinely frightening combination. Suppliers have obligations around affordable repayment arrangements, and you should not agree to a payment plan for a sum you are actively disputing without saying in writing that you dispute it.

If the debt is causing real hardship, speak to a free debt advice service before agreeing to anything. Citizens Advice runs the Consumer Helpline, and StepChange and National Debtline both give free advice on energy debt.

The short version

Find the date of your first accurate bill. Count back 12 months. If the supplier is the reason you were not billed before that, the charges for the earlier period should not be there — and the rule that says so is a condition of their licence, not a favour.