Your rights when your supplier charges for old energy use
Key right
Your supplier can only charge for energy used in the last 12 months if the billing error was their fault. All older charges must be written off.
Back-billing happens when your energy supplier sends you a bill for energy you used in the past — sometimes months or years ago. This usually occurs because your supplier failed to read your meter, applied the wrong tariff, or made an administrative error that was not caught at the time. When the error is eventually discovered, they try to recover the debt all at once, often with little warning.
Under Ofgem Standard Licence Condition 21B, your energy supplier is prohibited from charging you for energy used more than 12 months before the date they first notified you of the underpayment — provided the billing error was their fault. This rule applies to both electricity and gas suppliers licensed by Ofgem. It does not apply if you deliberately misled your supplier about your meter readings.
The 12-month rule applies when: the supplier failed to read your meter, the supplier applied the wrong tariff or meter rate, the supplier's systems incorrectly processed your meter readings, or the supplier made any other administrative error that caused the underbilling. It does NOT apply if you provided incorrect meter readings, tampered with the meter, or deliberately withheld information from your supplier.
Look at the disputed bill and identify the date your supplier claims the underbilling started. Count back exactly 12 months from when they first notified you of the problem. Any charges relating to consumption before that 12-month cutoff date must be written off. Your supplier must recalculate the bill using only the permitted 12-month window. If they cannot accurately calculate your actual usage for that period, they must use your consumption patterns to make a reasonable estimate.
If your supplier refuses to apply the 12-month back-billing cap, they are in breach of their Ofgem licence conditions. Write a formal complaint citing SLC 21B specifically. If they do not resolve it within 8 weeks (or issue a deadlock letter), you can escalate to the Energy Ombudsman for free. The Ombudsman's decision is binding on the supplier. In serious cases, Ofgem can impose fines on suppliers who systematically breach their licence conditions.
Any charges older than 12 months that result from your supplier's error must be written off entirely. There is no discretion — this is a regulatory requirement.
Gather your bills
Collect all bills covering the disputed period and identify exactly when the claimed underpayment began.
Calculate the 12-month cutoff
Work out 12 months back from the date your supplier first notified you. Any charges before this date must be written off.
Write a formal complaint
Write to your supplier citing Ofgem SLC 21B and stating the exact amount you believe should be written off. Use GL Complaint to generate a letter that cites the correct regulation.
Wait for a response
Your supplier must respond substantively within 14 days. If they do not, follow up in writing.
Escalate if needed
If your complaint is not resolved within 8 weeks, escalate to the Energy Ombudsman. Their decision is binding on the supplier.
Ready to complain?
Generate a professional complaint letter citing Ofgem SLC 21B in under 2 minutes. Free.
Start Your ComplaintFree escalation available. If your complaint is not resolved within 8 weeks, you can escalate to the Energy Ombudsman at no cost. Their decisions are binding on your supplier.