The Missouri Public Service Commission announced today that Ameren's (NYSE:AEE) regulated Missouri utility must return $26.3 million to its 1.2 million customers, as a result of not-so-clever bookkeeping, according to the regulators.

The Public Service Commission found that some of Ameren's power sale agreements to American Electric Power and Wabash Valley Power Assn. should have counted as sales for the company's fuel-adjustment cause, thereby offsetting higher fuel costs that were previously added in to higher rates for Ameren Missouri customers.

Ameren defended its decision, noting that its power sales were intended to make up for a sales stagnation from a large aluminum manufacturing customer, whose operations were knocked out by an ice storm for an extended period of time.

A previous review by the Missouri Public Service Commission for an earlier time period also determined Ameren to be at fault, forcing the utility to return $17.1 million to its customers.

Fool contributor Justin Loiseau has no position in any stocks mentionedbut he does use electricity. You can follow him on Twitter, @TMFJLo, and on Motley Fool CAPS, @TMFJLo.

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