The state’s largest utility is proposing to plug two batteries — really big batteries — into Oahu’s electricity grid to increase development, efficiency and cost savings of renewable energy production plants.

Hawaiian Electric Co. wants to establish the energy storage in Leeward Oahu at an estimated cost of $147.5 million and have the systems operational in about two years.

The two batteries combined would be capable of storing the equivalent of enough electricity to power 360 homes for a month based on the average Oahu home’s use of 500 kilowatt-hours a month.

If approved by the state Public Utilities Commission, the batteries would be the first large-scale power storage systems for the electric grid developed by Hawaiian Electric, though the utility company did test three relatively small 1-megawatt batteries on Oahu, Molokai and Hawaii island about two years ago.

HECO said the two lithium-ion batteries would pay for themselves and save ratepayers money by using renewable power that may otherwise be squandered instead of fossil fuels.

Currently, if solar and wind farms produce more energy than is needed to meet demand, the excess can go to waste. Peak production for solar and wind farms typically occurs in the middle of the day, while peak consumption of energy occurs at night. Storing energy produced by those farms during the day for use at night would allow HECO to use cheaper renewable energy instead of more expensive fossil fuels.

“System-wide everybody benefits because the more cheap renewable energy we can use, the less expensive oil we have to buy,” said HECO spokesman Peter Rosegg.

One battery would be able to store 80 megawatt-hours of energy and would be connected to a 20-megawatt HECO solar farm that recently broke ground near Pearl Harbor on Navy land. This solar farm’s size is capable of putting out 20 megawatts of electricity at any moment, and the battery can store about four hours worth of peak production from the farm for later use. HECO said the battery is eligible for a 30 percent federal tax credit because it would be connected to a solar farm, and estimates development cost at $43.5 million.

The other battery would be able to store 100 MWh of energy and would be built at HECO’s Campbell Industrial Park generation station, where it would receive power from the grid. HECO said the site was selected because there is no land cost, and infrastructure, including underground lines connected to an adjacent substation, already exist. HECO estimates the cost for this battery at $104 million.

Jeff Mikulina, executive director of Blue Planet Foundation, said his organization, which advocates for renewable energy, has been pushing the utility to incorporate more storage for a long time. “It’s good to see that Hawaiian Electric sees the value of adding utility-scale storage to the grid,” he said.

Mikulina, who noted that Kauai Island Utility Cooperative previously contracted with a few renewable energy developers to provide solar farms with battery storage, questioned whether a competitive bidding process might produce utility-scale batteries for Oahu’s grid at a lower cost.

HECO said it is also accepting proposals for new utility-scale renewable power generation that includes storage. The company is seeking 220 megawatts of renewable energy on Oahu, 60 megawatts for Maui and 20 megawatts for Hawaii island.

If HECO receives approval for the two batteries, it anticipates starting construction in late 2019 and finishing in 2020.