The probability that one of Hawaiian Electric Co.’s 17 generating units will be down for maintenance at any given time tripled in the past seven years as the fossil fuel-fired units age and struggle to compensate for the variable power produced by renewable energy.

HECO’s generating units are between 35 and 69 years old, not including the Campbell Industrial Park combustion turbine installed in 2009. The aging units require frequent and longer repairs, HECO said, which lead to a loss of reserve capacity on the grid.

“As the generating units age, they will need to be maintained more often and for longer periods of time,” HECO said in its annual Adequacy of Generation report. “Outages for planned work and maintenance will continue to be more numerous and longer in duration than in previous years.”

The added operating stress from renewable energy has caused more problems for reliability, as the old generating units struggle to ramp up and slow down more often to match the flows of solar and wind power.

The added strain can lead to outages.

“The generating units operate harder to counteract the increasingly dynamic changes, which increase the likelihood of unscheduled outages,” HECO said in the report filed with the Public Utilities Commission.

Issues with reliability might get worse for HECO, as the utility said it expects it will not have the necessary reserve capacity after 2018.

“Hawaiian Electric’s reserve capacity, which does not include intermittent energy sources such as wind and solar, may not be sufficient to meet the company’s generating system reliability guideline … in 2018 and beyond,” the utility said in the report.

In 2015 the probability that HECO’s generating units will not meet demand tripled from 2009.

“Right now the risk is all random in nature. It is a little like playing roulette,” said Matthias Fripp, assistant professor in the Department of Electrical Engineering at the University of Hawaii at Manoa. “Every day you simply spin the wheel and hope you don’t get double zero and lose two plants. This is an emergency and a rare event, but it is becoming less rare and we don’t have a lot of resources to respond to it.”

HECO’s Electric Equivalent Demand Forced Outage Rate, a measure of the probability that a generating unit will not be available due to forced outages, increased from 3.5 percent in 2009 to 10.2 percent in 2015.

“The higher that number gets, the greater the odds are that you will have other plants trip offline at the same time,” Fripp said. “They seem to be having trouble as they show more and more outages at their power plants.”

HECO said in the report that the utility might seek to remedy future capacity needs in 2018 and beyond by putting off future deactivation of units, increasing demand moderation programs, optimizing maintenance schedules, reactivating units that are currently deactivated or acquiring additional capacity.

Fripp said more customer participation is needed, such as demand response programs, which help the utility control spikes in demand.

An example of demand response is HECO’s hot-water heater program, which residents can sign up for and get discounts. HECO then can cut power to hot-water heaters temporarily to help ease demand on the grid.

“We need to develop smarter demand-side approaches,” Fripp said. “Right now we have a pretty brittle system on the supply side. … A better path would be demand response, storage and smaller starting power plants.”

Fripp said a renewable-energy system could handle a situation that led to blackouts in March much better than HECO’s current fleet of generators.

“If anything, I would expect a well-designed high-renewable power system to have fewer of these kinds of events, for two reasons,” Fripp said. “Every day, you are going to have solar coming off the system. When you expect that every day, we’ll be more adept to deal with it. … If we are relying on that much solar, it will be necessary to develop a lot of adaptive capability.”

HECO asked the state March 31 to approve a $340 million smart-grid system that would improve reliability, allow for increased use of renewable energy sources and give customers more control over their electrical use.

The smart-grid project would improve the system’s ability to detect outages and restore power. In addition, smart meters would provide customers with electricity usage information to help them better manage their bills. Customers would be able to see their personal energy information on their computers and mobile devices, to give them more control over their energy use and electrical bills.

Makena Coffman, associate professor for the Department of Urban Regional Planning at the University of Hawaii at Manoa, said a smart-grid program would be helpful in getting customers to participate in their energy use.

“The devil is in the details, but conceptually the point is to get down to a level where consumers and their appliances can understand what the loads are,” Coffman said. “This will be really critical for time-of-use rates.”

Fripp said dynamic pricing — pricing electricity higher when there isn’t a lot of electric production and paying customers to provide reserves — would be beneficial.

“As you offer customers the options of dynamic pricing, pretty soon after you will start getting customers that put in smart water heaters, smart air conditioners, smart lighting systems that can retrieve the price situation and react to it,” Fripp said.