Back Billing vs Catch-Up Bill
People weaken complaints by calling every large bill a back bill. Use this page to identify whether the supplier is recovering old charges or correcting previous estimates.
Use together
Difference table
| Question | Back billing | Catch-up bill |
|---|---|---|
| What is it? | A supplier trying to recover charges for older energy that was not billed correctly. | A larger bill caused by replacing estimates with actual readings or correcting underpayment. |
| Main evidence | Bill date, supply period, statement history and old usage period. | Reading history, estimated vs actual readings and usage pattern. |
| Best wording | Ask how the 12-month rule applies and what exception is relied on. | Ask for a corrected reading history and calculation breakdown. |
| Likely remedy | Remove protected old charges or rebill eligible period. | Rebill accurately, explain balance and offer payment support if needed. |
Decide which one you have in two minutes
Take the date printed on the bill and subtract twelve months. Write that date at the top of the page. Now look at the supply periods the bill covers.
Charges start after that date
This is a catch-up
bill. The supplier has replaced estimates with an actual reading and the balance jumped. It is almost
certainly payable. Your questions are about the accuracy of the reading and about
affordability — not about whether they are allowed to charge it.
Go to estimated vs actual readings.
Charges start before that date
Part of this bill
is potentially protected. Your questions are about why the supplier did not bill accurately at the
time. The amount being large is irrelevant; the billing failure is the point.
Go to back-billing rules.
Most large bills that arrive out of nowhere are a mix of both, which is why arguing the total gets nowhere. Split the balance in your first letter and the supplier has to engage with each half separately.
A worked example with real numbers
Abstract advice loses arguments. Here is the arithmetic a supplier has to be able to reproduce, using the Ofgem price cap rates for 1 July to 30 September 2026 (electricity 26.11p per kWh plus a 57.19p daily standing charge).
| Period on the bill | Units charged | At 26.11p/kWh | Standing charge | Line total |
|---|---|---|---|---|
| Apr 2024 – Mar 2025 older than 12 months | 2,900 kWh | £757.19 | 365 × 57.19p = £208.74 | £965.93 |
| Apr 2025 – Mar 2026 | 2,750 kWh | £718.03 | 365 × 57.19p = £208.74 | £926.77 |
| Apr 2026 – Aug 2026 | 950 kWh | £248.05 | 153 × 57.19p = £87.50 | £335.55 |
On a bill dated August 2026, the first row is the one that matters. It is roughly £966 of a £2,228 balance, it sits outside the 12-month window, and if the supplier failed to bill accurately during it, that line should come off. The other two rows are almost certainly payable — and saying so in writing is what makes the rest of the letter credible.
Why the standing charge line matters. People argue about units and forget that a full year of standing charge at 57.19p a day is £208.74 on electricity alone, before a single kWh. If a protected period is removed, its standing charges go with it. Ask for that explicitly, because it is frequently left on the bill.
Rates shown are the Ofgem cap averages for direct debit customers in England, Scotland and Wales, including VAT. Your region and payment method change them. Use the kWh to cost calculator with the rates printed on your own bill.
The two complaints, side by side
| Back billing | Catch-up bill | |
|---|---|---|
| Core question | Why did you not bill me at the time? | Is this reading and calculation correct? |
| The rule | SLC 21BA, 12-month limit. | No time limit. Accuracy and fair treatment. |
| Key evidence | Bill and statement dates, reading-request history. | Reading history, meter photos, serial number, usage pattern. |
| Best outcome | Protected charges removed from the balance entirely. | Corrected rebill, plus a payment plan spread over at least as long as the debt built up. |
| Realistic outcome | Partial write-off where the supplier cannot evidence its reading requests. | Small correction plus an interest-free plan. Goodwill for the poor communication. |
| Wrong move | Arguing affordability. That concedes the charge is valid. | Arguing the 12-month rule. It does not apply and it makes the rest look uninformed. |
On payment plans. If the bill turns out to be a valid catch-up bill, a reasonable position is to ask to spread it over at least the same length of time the arrears took to build. If under-billing ran for fourteen months, a fourteen-month plan is a proportionate ask, and it is much easier to get in writing than a write-off.
Five things that sink these complaints
Disputing the whole balance
If you refuse to pay anything, the argument becomes about your payment behaviour instead of their billing failure. Name the undisputed amount and keep paying it.
Arguing on the phone
Phone notes are the supplier's version of events. The Ombudsman reads documents. Every substantive point goes in writing.
Calling it unfair rather than unbilled
“This is unaffordable” is a payment-plan conversation. “You did not bill me for this period and the rule prevents you recovering it” is a back-billing conversation. They have different outcomes.
Missing the standing charges
Removing a protected period should remove its daily standing charges too. On a two-fuel account that is over £300 a year.
Escalating too early
The Ombudsman will send you back if you have not given the supplier eight weeks or received a deadlock letter. That wastes a month.
What works instead
A dated timeline, a numbered list of requests, a named undisputed figure, and a clear statement that you will escalate on a specific date. That combination gets escalated internally rather than parked.
Frequently asked questions
Can one bill contain both?
Yes. A bill can include recent corrected usage and older charges. Ask for a period-by-period breakdown.
Which page should I use first?
If the bill covers usage more than 12 months old, start with the back-billing rules page. If it is mainly estimate correction, start with estimated readings.
Can I still ask for a payment plan?
Yes. A valid catch-up bill can still be unaffordable, but affordability is a different issue from whether the charge is allowed.
What if the bill contains both?
That is the normal case. Ask for a breakdown by supply period, then write one letter with two clearly separated sections. Suppliers handle a split complaint far better than a single angry one about the total.
Does a catch-up bill mean I have no options at all?
No. You can still challenge the accuracy of the reading, ask for the calculation, and ask for a payment plan spread over at least as long as the under-billing lasted. You just cannot use the 12-month rule.
Official sources used for this page
BillDecoded translates official process and billing information into practical checks. It is not affiliated with the Ombudsman, Ofgem, Citizens Advice, Which? or any supplier.