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Back-billing calculator: how much of your catch-up bill is too old?

Enter the date of your catch-up bill and the period it covers. The calculator finds the 12-month line under Ofgem's back-billing rule and estimates how much of the bill is for older energy. It runs in your browser; nothing is sent anywhere.

Great Britain · Rules checked 24 October 2026 · Not legal advice

This tool gives general pointers, not legal advice. It can't see your bills or your supplier's records. Free help: Citizens Advice (England and Wales, 0808 223 1133) or energyadvice.scot (Scotland, 0808 196 8660).

How the 12-month rule works

Standard Licence Condition 21BA of every gas and electricity supply licence says that when a supplier bills you, or otherwise tries to recover charges (including through a prepayment meter), it may only charge for energy that could reasonably be considered to have been used, and standing charges built up, within the 12 months before the date of that bill or action. See Ofgem's back-bill guidance.

What the calculator does

Finds the 12-month line: the same date one year before the bill. A bill dated 29 February uses 28 February the year before.
Counts how many days of the period charged fall before that line.
If you enter an amount, it splits it evenly by day to give a rough figure for the part that could be removed.

Caveats

Seasons: you use more energy in winter, so an even split by day is only a guide. Ask the supplier for a breakdown by meter reading.
Earlier correct bills: if the supplier already sent you a correct bill for the older period and you didn't pay, the rule won't remove it.
Unreasonable behaviour: the rule doesn't apply if you obstructed accurate billing, for example by refusing meter access. Not sending readings isn't enough on its own (Ofgem, 2020).
Where: Great Britain only. Northern Ireland has different rules.

Questions

Which date counts as the bill date?

The rule looks at the date the supplier took 'charge recovery action', which usually means the date the bill was issued. If it first asked for the money another way, such as raising your Direct Debit or adding debt to a prepayment meter, use that date.

Do I have to pay the last 12 months?

Usually yes, if the readings are right. You can ask for an affordable repayment plan based on what you can afford (SLC 27.8).

Does this include standing charges?

Yes. Standing charges and other supply charges built up more than 12 months before the bill are covered too.

What if the supplier says I was obstructive?

Ask it to say exactly what you did and to send its evidence: dates of reading requests, visits and letters. A failure to give readings doesn't, on its own, take the supplier outside the rule.

Does it apply to businesses?

It applies to microbusinesses as well as households.