Hawaiian Electric Co. says it’s ahead of schedule in reaching the state’s renewable energy goal, largely due to the popularity of rooftop solar systems.

The state’s dominant electric utility said Wednesday that 25.8 percent of its electricity came from renewable energy resources last year, up from 23.2 percent in 2015.

HECO submitted its update to the state Public Utilities Commission on Tuesday, showing its progress toward meeting the state’s goal of having 100 percent of its electricity come from renewable sources by 2045.

The utility is ahead of schedule, with the next intermediate goal of 30 percent renewable by 2020 already within reach. HECO easily beat the 2015 benchmark, which mandated 15 percent of its electricity come from renewables.

HECO cautioned that adding renewable power to the grid will get more difficult as the state nears the 100 percent mark.

“Anyone who has run a marathon knows the first quarter of the course is easier than the last quarter,” said Peter Rosegg, spokesman for HECO. “But if you don’t make your time goal in the first quarter, it is even harder to finish within a deadline.”

Reaching nearly 26 renewable power is a major accomplishment, said Richard Wallsgrove, policy director at Blue Planet Foundation, a clean-energy nonprofit. The transition to renewable energy over the next 30 years will save the state $6 billion in avoided fossil fuel costs, he said.

Hawaii island is leading the race to 100 percent renewable.

About 54 percent of electricity used by customers on the Big Island came from renewables last year, up from 49 percent in 2015. In Maui County, 37 percent of the electricity customers used came from renewables, up from 35 percent in 2015. On Oahu, 19 percent of electricity used by customers in 2016 came from renewable resources, up from 17 percent the year before.

Kauai Island Utility Cooperative said the island hit 40 percent renewable energy dependence on Tuesday. The cooperative’s board recently changed its goal to have 70 percent renewable energy resource dependence by 2030, up from the original benchmark of 50 percent.

Across the five islands in HECO’s service areas, the No. 1 renewable energy source is “customer-sited” power, which is mostly rooftop solar systems on homes, offices, warehouses and parking garages plus a small amount of wind. HECO also gets renewable power from large, or utility-scale, solar and wind systems, geothermal, biomass, biofuels and hydroelectricity.

A major driver for renewable growth in the state is rooftop solar, making up 34 percent of the total amount of renewable energy connected to HECO’s grid.

Robert Harris, spokesperson for The Alliance for Solar Choice, said the data show HECO’s customers are leading the charge in getting to the state’s target.

“It is the customer that, ultimately, will ensure our clean-energy future,” Harris said.

Total energy from customer-sited systems attached to HECO’s grid rose 21.8 percent in 2016 from 2015. Meanwhile, renewable energy from all sources rose 4.4 percent last year at HECO.

Rooftop solar played a key role, despite the industry shrinking after the state ended a popular solar incentive program in 2015. The number of building permits issued for rooftop solar projects by the City and County of Honolulu was down 34 percent in 2016 compared with the year prior.

Wallsgrove said finding new ways to connect customer solar is important as the rooftop solar will play a key role in getting Hawaii to 100 percent renewable.

“The rooftop solar portion is important and something we can’t afford to slow down,” Wallsgrove said. “We know there is a lot more appetite for rooftop solar.”

Rosegg said the utility is betting on rooftop solar doubling in the years ahead. The utility has also issued searches for more wind energy projects on Oahu, to take advantage of federal tax credits.

By 2020, HECO expects renewables will make up 48 percent of the power mix — with more than 15 percent of that coming from customer-sited renewables.

In December, HECO said it expects 42 percent of homes, or 165,000 properties, in its service area will have solar systems by 2030 and that the utility will reach the state’s goal of 100 percent renewable energy five years ahead of schedule.

In the next five years, HECO said, it plans to add 360 megawatts of utility-scale solar, 157 megawatts of utility-scale wind and 115 megawatts from demand-response programs, which encourage customers to use electricity when more renewable energy is on the grid.

Rosegg said some of the technology HECO is planning to use “will change, but our goal and our determination will not.”