BOISE, Idaho (AP) -- Idaho Power Company is proposing an 8.5 percent increase to its monthly power bills, citing a variety of increased operating costs.

As part of its annual proposed rate adjustment, the state's largest utility is proposing an overall 7.1 percent rate hike for residential customers. That means the average monthly bill would increase $6, from $83 to $89 per month.

One reason for the proposed increase is a $43 million deficit in the so-called power cost adjustment. Idaho Power says federal requirements for buying alternative energy at cheaper rates is one cause for the deficit.

The utility is also seeking more revenue to pay for the $398 million Langley Gulch power plant fueled by natural gas.

The Public Utilities Commission is taking comments on the proposed increase until May 15.

Copyright 2012 The AP.