The Public Utility Commission began Wednesday to investigate consumer complaints that National Power Co. went back on a promise to electricity customers of a low, fixed rate.

The Houston company offered a price around 11 cents per kilowatt-hour for one year, but last week it informed some customers, without explanation, that rates would rise to around 15 cents.

The company invoked a clause in its contract that allows it to change the rate by giving customers 45 days' notice. That clause is standard in retail electricity contracts, calling into question whether customers can ever count on a fixed rate.

National's move is "bad for the market, it's bad for customers, it's just bad. It's bad all over," said PUC Chairman Barry Smitherman.

National Power chief financial officer David Barrett said in an e-mail that the company will meet with PUC officials Thursday and issue a statement afterward. He didn't return phone calls.

The commission also removed National's offers from the PUC's powertochoose.com Web site, where consumers can compare electricity prices.

The commission received nearly 200 complaints from National customers since the company mailed notice of the rate change on May 6.

The letters invoke the "material change" clause in the company's contract but don't explain what change prompted the rate hike. The company gave customers about a month to terminate the contract without having to pay the usual $300 penalty.

This angered one particularly high-profile customer.

"Give me a break, this is not a material change," said Steve Wolens, a former Texas legislator who wrote the electricity deregulation laws, which allowed retail electricity sellers to set their own rates.

A material change is an unforeseen event, he said, such as serious damage to transmission lines or power plant shutdowns. Mr. Wolens, a partner in the Dallas office of law firm Diamond McCarthy, said changes in wholesale power prices are a given.

National hasn't said why it raised the rate.

The "material change" clause mimics PUC regulations, which state that customers must be given 45 days' notice of changes to their electricity plans.

Most retailers lock in their electricity purchases months, even years, in advance.

So a bump in wholesale prices wouldn't alter profit or prompt a rate hike.

TXU Energy spokeswoman Sophia Stoller said TXU would only declare a material change to accommodate new laws or regulations.

She said the cheapest fixed price right now is TXU Energy's old regulated rate. The company cut that price last year to 12.4 cents per kilowatt-hour to appease legislators and promised to hold it through 2008.

Customers who switched from the old rate cannot switch back.