Hawaiian Electric Co. has sent letters to Oahu customers who applied for a popular solar incentive program before it was canceled, saying they need to pay an additional fee for a possible technical upgrade due to safety issues.

HECO said in a filing with the state Public Utilities Commission on Thursday that up to 1,100 customers could receive letters notifying them that they might need to pay for upgrades to qualify for net energy metering, a solar incentive program that credits customers the full retail rate for power sent to the grid. The 1,100 are some of the last customers who are eligible for the NEM program, which ended Oct. 13. A total of 1,175 customers applied before the deadline for the NEM program and are waiting to be connected.

HECO said the NEM customers’ service addresses failed certain mandatory safety screens because of over-voltage.

“Higher than expected voltage from excessive electricity output can trip off rooftop solar systems, damage home appliances, and could endanger utility crews who have to work on the electric grid,” HECO said. Because the solution will be tailored to a specific system, HECO said customers will need to pay for any costs associated with the upgrade. The utility did not say how much customers might have to pay.

The electrical utility will send NEM customers a status update within 60 days. The update could include a solution and the associated cost, HECO said.

HECO also said it will send out a letter with similar concerns notifying 66 customers waiting for approval for a solar incentive program called grid-supply. These customers will have to pay $1,700 for a technical review because they were flagged for potential over-voltage in an earlier screening.

Grid-supply customers have to notify HECO within 15 days if they want to move forward with the study and have to pay for it within 30 days. A total of 418 HECO customers have applied for grid-supply solar systems.

The customers could also switch to a third solar program, called “self-supply,” and not have to pay for the study. The self-supply system requires owners to link to a battery system and does not send excess power to the grid.

The letters to NEM customers were sent out Friday. HECO said it plans to mail letters to grid-supply customers Monday.

HECO said that it would update the customers on their status soon and provide another update in 60 days.

Some solar companies said they were hearing from concerned customers.

“It is imperative HECO discloses the work and costs involved in these studies and upgrades,” said Rich Taylor, project developer for Photonworks Engineering. “It should be mandated that they provide concrete evidence for the necessity of what they are proposing.”

PUC Chairman Randy Iwase said the agency sent HECO a letter asking the utility to explain the technical issues. The PUC also asked how many of the customers will be getting the letters.

“We’re just going to wait to see what kinds of technical, reliability and safety issues Hawaiian Electric is basing their further review off,” Iwase said.

HECO can refuse to connect solar systems to the grid if there are technical or reliability issues and ask customers to pay for a technical study, Iwase said.