If your supplier suddenly "discovers" a year and more of unbilled energy and wants it now, the back-billing rule is your shield: where the failure to bill was their doing, anything older than 12 months is generally not chargeable at all.
Suppliers' licence conditions bar them from recovering charges for energy consumed more than 12 months before the bill, where the delay was their fault: meter readings ignored, accounts unbilled, estimates never corrected. The rule exists precisely because surprise catch-up bills cause genuine hardship.
It protects you when you did the normal things: gave readings when asked, paid bills that came, didn't block the meter reader. It won't protect deliberate avoidance or tampering. Disputed bills also can't just be dumped into debt collection while you're actively challenging them.
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Check where I stand, free →This guide covers England and Wales; Scottish and Northern Irish rules can differ. This is guidance to help you understand your rights, not formal legal advice, and we're entirely independent: no affiliation with Citizens Advice, GOV.UK or any law firm. Every quotation above links to the official text so you can check us. Law verified current at July 2026.