State regulators have approved Hawaiian Electric Co.’s request to buy as much as 1 million gallons of biodiesel a year to run an emergency power generation system at Honolulu International Airport.

The fuel will be supplied by Maui-based Pacific Biodiesel, which makes biodiesel from waste

cooking oil.

Under the three-year agreement Pacific Biodiesel will supply HECO with 250,000 to 1 million gallons of biodiesel a year to be burned at a new 10-megawatt power plant being built next to the utility’s airport substation.

The $20 million generation facility features four diesel generators and is expected to begin operations next year.

HECO did not disclose the price it will pay or the biodiesel under the contract approved Dec. 13 by the Public Utilities Commission. HECO officials said in a PUC filing that the price would be based on “the average of three markets to arrive at a balanced price.”