Hawaii electricity grids are about to be connected for two-way integration with battery storage systems in homes across the state.

A company Thursday began accepting applications from Hawaiian Electric customers interested in earning fees for allowing existing and new rooftop solar system batteries to help modulate electricity supply and demand on Oahu, Maui and Hawaii island.

Essentially, a residential customer’s battery could be used to absorb excess renewable energy from the utility so that a grid isn’t overloaded, or be tapped as a source of electricity if a grid needs more supply.

California-based Swell Energy Inc. seeks to enroll 6,000 Hawaiian Electric customers on the three islands in its “Home Battery Rewards” program.

The goal of the program, which was approved in January by the state Public Utilities Commission, is to turn networks of customer rooftop solar battery systems into manageable energy banks for Hawaiian Electric grids where electricity generation and use need to balance.

Too much generation, especially from wind and solar power during the day when electricity use is lower, can disrupt a grid. On the flip side, evening peak energy demand needs to be served after the sun has set.

“Our islands have small, stand-alone grids with a high-level renewable generation, which makes them sensitive to supply and demand imbalances,” Yoh Kawanami, Hawaiian Electric customer energy resources co-director, said earlier this year in a statement announcing the approval of the contract with Swell.

The state’s largest regulated utility expects that over the coming 20 years it will need many more customers and independent companies participating in Hawaii’s expanding clean-­energy future with their own generation, storage and participation in such “grid serv­ice” partnerships managed by third-party technology contractors like Swell.

Last month Hawaiian Electric published a request for proposals from grid serv­ice providers to aggregate what it calls “customer-­ sited distributed energy resources” of up to 60 mega­watts under a 10-year management contract similar to what Swell has begun to do.

Swell’s program is for up to 80 megawatts, which equates to about 4,000 residential battery systems on Oahu, 1,500 on Hawaii island and 500 on Maui.

The company is initially working with equipment installation partners Revolu­Sun on Oahu and Rising Sun Solar on Maui and Hawaii island but expects to add others.

Under Swell’s program, participants earn a fixed monthly fee based on battery capacity.

Swell expects this monthly fee to be about $40 on average for a typical single battery, with any additional batteries increasing the payment by $40 a month per battery. Such payment would appear as a credit on a customer’s electricity bill, with any excess credit paid in cash.

Suleman Khan, Swell’s CEO, said the energy management system is designed to prioritize electricity value to the battery owner while delivering flexibility to the grid.

“It is truly breaking the glass ceiling of renewables,” he said.

Swell’s contract is for five years and is expected to cost utility ratepayers $25 million to pay program participants and Swell.

Kawanami said Thursday in a statement that Hawaiian Electric customers can now get more value from battery systems with grid service programs like Swell’s.

“We are at a turning point in Hawaii for advancing the capabilities of all residential solar and batteries already deployed and being installed in the future,” he said. “By harnessing the collective power in our homes, we enable a more affordable and resilient Hawaii that also helps us meet our 100% renewable energy goals by 2045 or sooner.”

Some of the near-term effort to harness residential rooftop solar battery systems for managing electricity on Oahu is being driven by the need to avoid a potential shortage when a coal-fired power plant owned by AES Corp. is supposed to be retired a year from now.

This 180-megawatt plant at Campbell Industrial Park provides around 16% of Oahu’s electricity during peak evening hours.

Another company that has a contract with Hawaiian Electric to aggregate so-called customer energy resources is Open Access Technology Inc., which has been working with Sunrun, a solar energy provider, on a program similar to Swell’s.

Hawaiian Electric in July also launched its own program to encourage customers to add batteries to rooftop solar systems. This program, dubbed “Battery Bonus,” offers customers a one-time payment between $500 and $850 per kilowatt of new battery storage so that the utility company can ensure the battery system is powering the home or supplying the grid for two hours within the peak use period of 6 to 8:30 p.m. over 10 years.

The utility’s program aims to add as much as 50 megawatts of battery storage and provide customers with up to $34 million in incentive payments paid for by utility ratepayers.