Utah Power & Light Co., riding a recent wave of record power demand and reaping benefits from decreased operating costs, reported improved earnings for the first nine months of 1988 compared with the same period last year.

Earnings for the nine month period ending Sept. 30 were $125.58 million, or $2.09 per share, on operating revenues of $777.22 million. For the first nine months of 1987 earnings were $105.25 million, or $1.73 per share, on operating revenues of $740.78 million.Earnings per share for the last 12 months were $2.75, compared to $1.64 after the same period a year ago.

For the third quarter, UP&L had earnings of $49.90 million, or 83 cents a share, on operating revenues of $277.18 million, compared to the 1987 third period performance of $43.37 million in net income, or 72 cents a share, on revenues of $250.26 million.

UP&L spokesman Dave Mead said revenues were up 3 percent in residential service, 3 percent in commercial and 18 percent in industrial and irrigation for the 12 months ending Sept. 30 because of an early summer heat wave and a slight increase (3,800 new ratepayers) in customer growth.

In June, UP&L had an unofficial record demand of 2,584,000 kilowatts, exceeding the previous peak demand set in 1984. The early summer heat wave and drought contributed to the extra demand, Mead said, as the utility's irrigation demand peaked one month earlier than usual.

Cost cutting efforts and tax breaks also contributed to this year's improved performance. A program to refinance the utility's high interest debt has saved $2 million in operating costs this year, Mead said, and an unspecified drop in taxes resulted in savings of about $15 million.

Mead said UP&L's central Utah coal operations have also cuts costs. Coal production has increased 14 percent in the last 12 months, while mining costs have risen just 6 percent.