Few people doubt, at least publicly, that Hawaii will be able to fulfill Gov. David Ige's vision to rely entirely on renewable power generation sources by 2045.
At the same time, few people know exactly how the utilities that serve the islands will accomplish that tall order, or how many millions of dollars it might cost them and their ratepayers.
"I think, yeah, it can be doable, but how is it going to be done exactly? That's the big question. It's very hard to say," said Anthony Kuh, an electrical engineering professor and the director of the Renewable Energy and Island Sustainability Center at the University of Hawaii at Manoa.
Hawaii has long been on the renewable-energy vanguard by necessity. The state has no fossil fuel and relies on imported oil for its power plants, giving it the country's highest electric rates.
Hawaii's goal was to have 15 percent of its power be renewable this year, and it will beat that easily. Honolulu-based Hawaiian Electric Industries Inc., which provides electricity to five Hawaiian islands, said that in 2014, 21 percent of its power was renewable. Kauai Island Utility Cooperative, which is based in Lihue and serves the island of Kauai, expects to get 38 percent of its power from renewable sources this year.
But getting to 100-percent will take much more than merely doing more of the same.
To its good fortune, Hawaii has a wealth of renewable energy sources to choose from, including some that many other states can only dream of.
Its most exotic is geothermal energy, which it can tap due to its location on a volcanic archipelago. The state only has one geothermal plant, which produced a quarter of the island of Hawaii's electricity last year. Nonetheless, according to estimates made in the 1970s and '80s, it has enough geothermal energy to power all of the islands of Hawaii and Maui and 60 percent of the electricity consumed by Oahu.
The state also is the site of pilot projects aimed at getting energy from the ocean, including one that uses the temperature difference between deep and surface water, and another that uses waves.
Hawaii's climate also makes the state ideal for wind and solar power.
A 2011 study by the National Renewable Energy Laboratory found that Hawaii could generate 12.4 million megawatt hours of wind power annually. That's not terribly far from the 10.3 million megawatt hours of power generated in the state in 2013, according to the U.S. Energy Information Administration.
Hawaii hasn't moved rapidly to capture more wind power, however. The state only has 206 megawatts of installed wind capacity, ranking it 29th, according to the American Wind Energy Association. That said, Hawaii has four wind farms and plans two more - a 24-megawatt one on Oahu and a 60-megawatt one on Maui.
Hawaiians have embraced solar power to a greater degree, with tens of thousands of rooftop and commercial solar installations.
HEI said earlier this year that 12 percent of its customers on Oahu, 10 percent of its customers on Maui, Molokai and Lanai, and 9 percent of its customers on the island of Hawaii have rooftop solar units. By comparison, only 0.56 percent of electric customers nationally have rooftop photovoltaic systems, according to data from the Solar Electric Power Association.
While it would seem to help Hawaii move toward its renewable power goal, the state's high adoption rate of rooftop solar also highlights some of the obstacles to getting there.
For instance, the large number of rooftop solar units led HEI to stop connecting new ones in neighborhoods with a lot of them already, for fear its grid wouldn't be able to handle all the power from them. The utility came out with a plan to address the issue last November.
The large number of residential customers with solar units also led HEI to seek approval from the Hawaii Public Utility Commission to sever the link between the amount of power it sells and its total revenue, so it doesn't have an incentive to discourage rooftop solar units, which reduce the amount of electricity its customers buy.
Additionally, in June, HEI asked the PUC to allow it to implement a new pricing structure for new residential solar units so that its solar customers could begin to pay their fair share of its grid operations and maintenance costs. The company said customers with rooftop solar units are shifting about $53 million of those costs each year to customers who don't have them.
HEI also asked the PUC to let it charge a minimum monthly bill of $25 for future residential solar units on all islands - up from $17 on Oahu, $18 on Maui and $20.50 on the island of Hawaii - and to approve a trial time-of-use rate for customers with new PV systems that would encourage them to shift their power consumption to times of day when the systems generate the most electricity.
In September, the Kauai utility got permission from the PUC for a test program in which it will offer 300 residents with smart meters a 25 percent discount on power consumed between 9 a.m. and 3 p.m. The program is meant to prevent the utility from having an oversupply of power from its solar resources during the day and to enable it to reduce the amount of power it provides at night from its oil-fired generators.
Hawaii, meanwhile, is embracing solar power in places other than its residents' roofs.
The state PUC in August approved power purchase agreements for four solar farms with 137.2 megawatts of generating capacity on Oahu. The developers of the farms want to get them built ahead of the 2017 expiration of a program that provides investors in the projects with 30 percent federal tax credits.
More such projects will be needed for the state to meet its renewable energy goal, along with large-scale battery storage, which some say will require government support.
"We're still in the infancy of storage and if we're going to get 100-percent renewable we need to make sure that the storage component of a project is subsidized," said Drew Bradley, a regional manager at REC Solar, which has installed more than 30 MW of solar projects in Hawaii in the past eight years.
Other, more dramatic changes will be needed as well.
Currently, each island has its own grid. To get to 100-percent renewable power, they may have to be connected.
That won't be easy or cheap.
An interconnection to distribute electricity from a large wind-power project planned for Molokai and Lanai was proposed, but those islands' residents objected and the idea was shelved, according to Jeff Mikulina, the CEO of the Blue Planet Foundation.
Blue Planet was the lead advocate of Hawaii's commitment to getting all its power from renewable sources, but even Mikulina concedes that the Honolulu-based nonprofit isn't sure how the state will achieve its goal.
"We really don't know what the landscape's going to look like" in 2045, he said.
Mikulina thinks Hawaii will need to do at least two things to meet its renewable-power mandate: Continue to be an early adopter of renewable technologies, and change the operation of its electric utilities to make them more like integrators of various technologies, including distributed generation, demand-response programs, power storage, and electric cars that can return energy to, as well as take energy from, the grid.
That kind of shift will pose myriad challenges for HEI, which serves 95 percent of Hawaii's residents, although the utility has begun getting ready for it.
In addition to beginning to tweak its revenue model, HEI, which may be bought by Juno Beach, Fla.-based NextEra Energy Inc., has launched a smart-grid program that it plans to extend across the state.
Still, like all electric utilities, HEI's primary function is to keep its power grids up and running, and that means it may have to adopt new technologies more slowly than utilities elsewhere.
"We have such small and sensitive grids that if you put something in and it really doesn't work well, you can have a much greater (negative) impact," said Scott Seu, VP of system operations for HEI's Hawaiian Electric subsidiary, which directly runs the Oahu grid and runs the grids on the islands of Hawaii, Maui, Lanai and Molokai through subsidiaries.
HEI and NextEra have pledged to meet Hawaii's renewable-energy goal and Seu thinks they can, but that doing so will require advances in technology for households, too, and not just utilities.
Residences, he said, would have to have sophisticated energy systems that manage power consumption and have a storage component. At the same time, utilities will need to have greater visibility into their grids and greater control over power-generation and demand-control resources down to the customer level.
"Most grid operators had not really thought about the need to control small distributed resources - there's just too many of them - but as we go forward into the future, that is going to become more critical," he said.
Complicating matters further, Maui residents are considering forming a municipal utility or a power cooperative, and residents of the islands of Molokai and Hawaii are considering forming power co-ops.
These concerns and others no doubt came up during a recent fact-finding trip to Hawaii by a group of energy executives led by SEPA.
The trip focused on a number of questions including whether state policies are helping and what needs to happen to "counteract any negative operational impacts" on utility revenues.
The foremost question for Hawaiians themselves, though, is whether they will see a drop in their electricity bills, which are about two and a half times the mainland average.
HEI CEO and President Alan Oshima thinks the answer to that is yes - but only to a point.
"There are costs for living in a disconnected, 2,500 miles-to-the-nearest-neighbor state," Oshima told Civil Beat, a Hawaii public affairs news site. "So let's be real!"