The 30 Utah utilities owning part of the Intermountain Power Project will share in more than $300 million left over from early completion of the plant, but not until they begin buying power, a project official said.

The 1,600-megawatt power plant was completed in June 1987 and has been selling all its output to six Los Angeles-area municipal utilities."Because the plant came in under budget and ahead of schedule, we're returning the surplus to the owners," IPP spokeswoman Ann Garrett said.

Intermountain Power Agency, which runs the plant for the three dozen municipal utility-owners, came up with "an equitable plan to divide the surplus in proportion" to each owner's share, Garrett said.

The surplus will be distributed in the form of credits when a member buys power from the $5.5 billion plant. None of the members are buying power now, but have "leased" their power shares to southern California utilities.

The Utah municipalities and rural utility cooperatives receive or generate adequate power without IPP, so they have "laid power off until they need it," said Garrett, allowing Los Angeles, Anaheim, Burbank, Glendale, Pasadena or Riverside to buy those Utah shares.

About $278 million has been earmarked as surplus and another $50 million will be set aside to credit member power purchasers, an official said.