Utah Power & Light Co.'s Utah ratepayers pay more for electricity than the utility's Idaho customers, who pay some of the lowest rates in the country, a national survey said.

The annual study by the National Association of Regulatory Utility Commissioners (NARUC) ranked the cost of residential power among 200 investor-owned utilities nationwide during the winter months of 1987-88.Capturing the number one spot with the highest-priced power in the country was Consolidated Edison Co. of New York. Its New York City customers pay an average 13.5 cents per kilowatt hour, or $67-$68 a month. The least expensive service territory was Lewiston, Idaho, served by Washington Water Power Co., paying just 4 cents per KWH.

The nationwide average was 8.04 cents KWH.

In Salt Lake City, UP&L customers paid an average 8.5 cents per KWH, the survey said. Power bills for 500 KWH during December, January and February were an average $42 per month. Those prices placed UP&L's Salt Lake market as the 68th most expensive in the country.

But UP&L's Rexburg, Idaho, customers live in one of the 25 lowest priced service areas, with a 177th ranking. Cost per kilowatt hour in Rexburg last winter was 5.8 cents and the monthly bill for 500 KWH was an average $28.

UP&L spokesman Dave Mead said the difference in cost between Idaho and Utah is because of federal regulations allowing UP&L to blend the cost of cheap federal hydro power with its expensive coal generated electricity to determining its Idaho rates. Under the Northwest Power Planning and Conservation Act of 1980, however, Utah rates don't receive that same treatment and are based solely on the more expensive coal generated power.

On a statewide average, Mead said, UP&L customers pay about 7.9 cents per KWH.

Despite the apparent low costs, customers in UP&L's Idaho service territory petitioned their Legislature two years ago to allow them to switch to less expensive Idaho Power Co., based in Boise. Idaho Power, which generates its electricity from hydro projects, offered the fourth lowest rates in the survey.

But, a recent Idaho Supreme Court ruling will change that ranking. The court said regulators must allow the utility to include the cost of a Nevada coal-fired generating plant in its rate base, which will force rates up by 8 percent.

Also among the low cost producers in the survey was Pacific Power & Light Co., which also has access to inexpensive hydro power. The utility's Washington, Oregon and Montana service areas were among the 25 least expensive in the survey.

Pacific Power's parent company, PacifiCorp, and UP&L want to merge and the companies have promised their marriage will lower rates for UP&L customers.

The $2 billion merger has been stalled, however, by a conditions laid down by federal regulators, delaying approval in Utah and prompting requests for rehearings at the Federal Energy Regulatory Commission in Washington, D.C.