Enmax customers will pay a special one-time bill this month and the money will be used to pay off a long-running mistake by the utility company.

The bills, which will mean almost $1 million in revenue for Enmax will be paid by Calgarians that still get their water on a flat rate as opposed to a water meter.

Helene Larocque got a letter with her last bill saying that Enmax had forgotten to charge her the increased 2012 flat water rate since January.

Now Enmax is back billing her for the amount she should have paid.

Larocque's extra charge is only $34 but she says it is the principle that bothers her.

"It's a general business practice, I mean if you make a mistake you absorb it, you don't expect the client to pay for it," said Larocque.

Laroque called our Consumer Specialist Lea Williams-Doherty for help.

Lea tried to find out if Enmax even has the authority to back bill for the error.

Lea and Laroque got several different answers to that question while making inquiries.

When Laroque called Enmax she asked the customer service rep “on what authority can you back bill me for your mistake?”

She says the rep told her it is in the terms and conditions of her contract.

Lea and Laroque couldn’t find anything about it in the contract so Lea emailed Enmax to tell them that.

Emax confirmed that no, it's not there and that the customer service rep was incorrect.

Lea went down to Enmax and asked “if my credit card company announced that it was gonna raise rates for 2012 but then forgot to put it on my bill for 9 months, I don't think they could come back and say I owe more on my principal, what is it that makes utility companies different?"

Enmax spokeperson Ian Todd said "Well it's the regulatory body that allows utility companies to go back and collect from consumers that underpaid on their accounts."

Lea then asked for a copy of that regulation, which would have been written by the Alberta Utilities Commission, and was told no, that was wrong too.

She was told that the authority is granted by the city's water utility bylaw.

Lea then asked for a copy of that document and after reading through it she found that there is relevant language in the bylaw.

The bylaw states that customers shall pay for water as per the approved rate schedule but there is no provision addressing Enmax’s rights and responsibilities if it bills customers incorrectly.

Still Emax interprets the bylaw to mean its mistakes do not relieve customers of their obligation to pay the approved rate for water used.

So if Enmax can come back and charge you for a bill you thought you'd already paid how long might you be on the hook for undiscovered mistakes?Enmax says for residential customers, the period is one year and for commercial customers it's two years.

Lea asked Enmax to show her the regulation that spells that out and found out that there is nothing written about that. Rather, the rule regarding the period of time during which Enmax is allowed to back bill its customers for mistakes is based on past city practice which has established a one-year period for residential utility customers.