Tesoro Corp., the operator of seven oil refineries in the western United States, reported a fourth-quarter loss after a power failure disrupted operations at its plant in Hawaii and processing margins narrowed.

The net loss of $40 million, or 29 cents a share, compared with net income of $158 million, or $1.14, a year earlier, the San Antonio-based company said Thursday. Sales rose 63 percent to $6.53 billion.

Tesoro has a Salt Lake City refinery and about 40 gas stations in the Salt Lake area. The company said its refinery in Kapolei, Hawaii, had a fourth- quarter pretax operating loss of $86 million, compared with profit of $19 million a year earlier.

Tesoro fell $1.50, or 3.7 percent, to close Thursday at $38.90. The stock has dropped 18 percent this year after a 45 percent gain in 2007.