Environmental and clean-energy organizations chastised Hawaiian Electric Co.’s proposal to generate more electricity from coal.

HECO wants to increase the electricity supplied by AES Hawaii — the only coal-fired plant on Oahu — to 189 megawatts from 180 megawatts, the electric utility said in a filing with the state Public Utilities Commission released late Monday.

“I’m disappointed but not surprised,” said Marti Townsend, director of the Sierra Club Hawaii Chapter, on Tuesday. “HECO continues to make shortsighted decisions that benefit their investors and harm the public and our shared environment.”

Coal releases more carbon dioxide than oil when burned to produce a given amount of electricity. Coal is generally considered cheaper than oil.

“What does this say about our society if we are limiting solar energy and increasing coal use?” Jeff Mikulina, executive director of Blue Planet Foundation, a clean-energy advocacy group, asked Tuesday.

HECO spokesman Darren Pai said the utility remains committed to achieving the state’s goal of 100 percent renewable electricity by 2045 and that Friday’s filing to add energy from coal would help make up for the fluctuating availability of renewable energy.

“As we work toward that goal, we’re asking the PUC to approve an amendment to the AES contract. (This) will mean lower bills and greater reliability for customers, while supporting the increasing amounts of intermittent renewable energy on the grid,” Pai said.

HECO would pay AES between 5.2 and 6.4 cents per kilowatt-hour for the added power. According to the PUC’s 2015 annual report, AES was the lowest-cost power producer in the state.

Pai said that the utility is not required to buy the additional energy from AES, but that it’s there only for backup.

“We’re not required to buy the additional amount of electricity, but it provides a cushion of firm energy should we need it over the remaining term of the current contract,” Pai said.

Last January, Oahu residents experienced rolling blackouts after AES unexpectedly went out of service along with one generating unit at the Kahe Power Plant. Another factor was lower output from the Kalaeloa Power Plant, which was undergoing maintenance at the time.

The amended contract with AES would give the producer the opportunity to earn a reliability bonus — beginning retroactively Oct. 1 — for each year the plant doesn’t trip offline.

The bonus would be $1 million if AES does not shut down at all, $700,000 for shutting down only once, $500,000 if the plant shuts down twice and $300,000 for shutting down three times.

The existing contract allows six unit shutdowns every contract year.

HECO said the additional 9 megawatts from AES will help replace a shortfall expected when the utility retires its oil-fired plants. The utility said its reserve capacity may not be enough as it continues with its plans to deactivate Waiau units 3 and 4 at the end of 2017. Waiau units 3 and 4 have a combined 92.6-megawatt capacity.

HECO said the changed contract also keeps open the possibility of AES including a biomass fuel service for the remainder of the term of the power purchasing agreement, which ends in 2022.

“This does not extend the life of the current AES contract,” Pai said. “As we transition to more renewables, the amendment offers lower prices and more reliability.”

Blue Planet Foundation is working to support legislation this session to discontinue HECO’s use of coal once the contract with AES ends.