When Katherine Ziemann got back into her Nissan Leaf after a three-week trip to Portugal, she drove out of a parking garage at Daniel K. Inouye International Airport without paying a dime.

Ziemann saved $440 because of an incentive signed into law by Gov. Neil Abercrombie in 2012. The law exempts owners of electric vehicles from paying parking fees at the airport.

Ziemann isn’t alone in taking advantage of the law. Over the last five years, EV owners have saved a combined $2.6 million while parking at the airport, according to the state Department of Transportation’s response to a Honolulu Star-Advertiser query.

EV owners are increasingly using the airport benefit. This year, through June, EV owners parked at the airport 18,224 times. From January 2013 to June of this year, EV owners have parked at the airport 87,676 times.

The airport parking benefit and other EV incentives were meant to help spur EV adoption and reduce the use of fossil fuels in Hawaii.

They have done just that, said Margaret Larson, state energy office analyst. “Incentives made available to electric vehicle owners under Act 168 have contributed to the growth of EVs in Hawaii, which supports the state’s energy policy,” Larson said.

In addition to free parking at the airport, EV owners don’t need to pay for parking at meters or at any state or county garage. They also are allowed to drive in high-occupancy vehicle (HOV) lanes without restriction.

As of June there were 5,926 EVs on the streets in Hawaii, an increase of 32.5 percent from the 4,472 registered in the same month last year. Still, EVs represent only 0.56 percent of all registered vehicles in the state.

But at the Honolulu airport, EVs make up a much larger portion of all parked cars.

On one floor of a garage at the airport on a Wednesday this month, there were 21 Nissan, Tesla and BMW electric cars out of 100 vehicles counted.

Supporters of the EV incentives say the $2.6 million EV owners saved in airport parking fees doesn’t equate to $2.6 million in lost state revenue because many of the EV drivers would not park at the airport if they had to pay.

Ziemann, the Nissan Leaf owner, said the opportunity to park for free at municipal lots and meters led to her buying her Leaf, and she never parked at the airport until she bought an electric vehicle.

“Before, I would have to arrange rides from friends or pay an Uber, which was inconvenient and costly,” she said. “I think the benefit is great, especially when you land, because you don’t have to wait on your ride or inconvenience your friends if the flight is delayed.”

Shem Lawlor, clean-transportation director at Blue Planet Foundation, said, “If the parking lot is actually full, then you can make a case that EVs are taking revenue from the state’s pocket.” Lawlor, who also owns an EV, said he was dropped off for his flights before switching from a gasoline vehicle.

There is some question as to whether Hawaii’s EV incentive law was intended to include free long-term parking at the airport. The law says EV owners are allowed to park for free at state or county parking lots unless the “fee is assessed in increments longer than one twenty-four-hour day, including weekly, monthly, or annual parking permits.”

The state DOT is interpreting the law to mean that electric vehicle owners can park at the airport free for up to 30 days.

“An electric vehicle on which an electric vehicle license plate is affixed is exempt from payment of parking fees,” the HDOT’s website reads. All vehicles left over 30 days at the airport are subject to towing.

“No budgets are being busted or anything like that because of this,” said Sen. Will Espero (D, Ewa Beach-Iroquois Point) when asked about the airport parking benefit. “This is part of the alternative energy initiative, and if we can get more EVs on the road, obviously we should do what we can and what is within our powers and authority to do.”

Espero said if there was a desire to change the incentives, discussion could go either way.

“If there is a need to revisit that figure, we’re always open and willing to do that,” Espero said. “If people want to raise it, if we want to (remove the incentive), let’s talk,” he said.

Kalihi resident Ezra Ami­stad doesn’t own an EV, but said she doesn’t mind EV owners benefiting because it helps shift the state off fossil fuels, which helps the environment.

“It makes such a bigger impact than just the Department of Transportation,” Amistad said. “As soon as the rail is completely finished, if they added that kind of benefit for electric cars (parking at rail stations), I think it would be awesome.”