Honolulu's planned commuter rail system will consume enough electricity each day to power about 9,250 homes, or a community the size of Hawai'i Kai.

That shouldn't pose a risk of energy shortage and could lower transportation-related air pollutants and energy use as people switch from automobiles to trains, according to the city.

The potential positive impact on the environment is a key benefit of the $3.7 billion elevated commuter rail line, said Honolulu Mayor Mufi Hannemann.

"We are already a community that is 90 percent dependent on fossil fuels and this is all about reducing our dependence on that and using alternative forms of energy," he said. "We're putting Honolulu in a good place to continue to do our part to reduce global warming."

Environmentalists agree that mass transit can be a green transportation alternative. Whether rail is more energy efficient and reduces air pollution hinges on factors such as ridership and the fuel used to generate electricity.

"Based on the experience of Mainland cities and cities around the world, it seems certain that rail has been a good option for most of those cities," said Randy Ching, chairman of The Sierra Club O'ahu Group. "I would hope it would be similar here, but I haven't seen definitive numbers either way yet.

"If I were convinced that this would be well used and a lot of people would get out of their automobiles to ride the rail, then I think there's no question on a cost per mile, per passenger basis rail would be a more viable option than the personal automobile," Ching said.

The city plans to start building the elevated commuter rail line late next year, with the first segment from East Kapolei to Leeward Community College opening in 2012. The total 20-mile route to Ala Moana would open in phases through 2018.

In 2030, the train system is expected to consume an average of about 18.5 megawatt hours of electricity, according to New York-based Parsons Brinckerhoff, the project contractor.

The power needed could exceed 18.5 megawatt hours because that estimate does not reflect the energy used in the generation and transmission of electricity by Hawaiian Electric Co.

The power needed by the train is relatively small when compared with O'ahu's total daily peak demand of about 1,241 megawatts of electricity, according to HECO.

HECO said there should be no problem supplying the train with power, along with other potential new projects such as new housing developments and the planned University of Hawai'i West O'ahu campus.

The train "is a big project obviously," said HECO spokesman Peter Rosegg. But, "It would be wrong to see (the train) as some big gorilla coming down the road. It is one of the small monkeys and there are a number of small monkeys and they're going to get here at different times."

While the train's demand for electricity could be relatively small, peak transit hours — and electrical needs — are likely to coincide with Honolulu's overall electricity demand peak, which occurs in the early evening.

The added load from the train also is expected to come at a time when HECO forecasts reserve capacity shortfalls of as much as 130 megawatts in 2014, according to a report filed this year with the Public Utilities Commission. That's assuming the utility completes a 110-megawatt generator at Campbell Industrial Park sometime next year.

A reserve capacity shortfall increases the risk of generation-related power outages. However, that is not expected to be an issue on O'ahu unless HECO's two largest power generators are taken offline for some reason. Even then, HECO would hope to manage demand to prevent an outage, Rosegg said.

"We can and will accommodate the energy needs of the transit program," Rosegg said. "These needs will not be the driver for another power plant."

To help meet electricity needs, HECO is looking at renewable energy sources, including wind, solar, waste-to-energy and ocean power, the company said. The utility's new Campbell Industrial Park generator will use biofuels, which HECO said will only come from farmed palm oil plantations that haven't been cut out of native forests.

Whether rail is more "green" than other transportation modes also depends on how many people opt to park their cars to ride mass transit. Critics have questioned whether a Honolulu rail line can generate high ridership without serving Waikiki, UH-Manoa and Honolulu International Airport. The city plans to eventually expand service to those locations, but there is no money earmarked for such expansion.

Even without serving those areas, the commuter rail line is expected to attract more riders when compared with other alternatives studied, including expanding TheBus fleet or creating managed traffic lanes, according to Parsons Brinckerhoff.

When combined with TheBus, overall mass transit ridership is expected to increase to 7.4 percent of total transit trips in 2030, up from 6.1 percent projected if the commuter rail line is not built. The transit system also is expected to reduce daily vehicle trips by 45,000 on O'ahu.

Higher mass transit use could mean daily transportation energy consumed in 2030 would be about 3 percent less than if the city were to build managed traffic lanes, according to Parsons Brinckerhoff. The energy needed to run the commuter rail system or an expanded TheBus fleet would be nearly the same.

Rail energy reductions are based on the higher energy efficiency and passenger load factors of rail transit. Commuter rail requires 2,743 Btu of energy per passenger mile, according to the Department of Energy. A Btu, or British Thermal Unit, is equivalent to the energy needed to raise 1 pound of water by 1 degree Fahrenheit. In contrast, an automobile uses 3,445 Btu of energy per passenger mile.

City officials argue that transit ridership could outstrip expectations as gasoline prices rise and traffic congestion grows. However, higher gasoline prices also are expected to spur the development and adoption of more fuel-efficient autos. That means that, some day, autos could be more energy efficient than rail.

By 2035, the average auto will consume an estimated 2,500 Btu per passenger mile, according to an April article published by the libertarian Cato Institute. "A proposed light rail line that promises to save energy not only needs to be more efficient than today's autos, it must be more efficient than future autos," wrote the article's author, Randal O'Toole.

Any evaluation of the energy savings generated by rail also needs to consider the massive amount of energy required during construction, according to the Cato report. For example, construction of the fixed guideway will require between 3.7 trillion and 4.9 trillion Btu of energy, according to Parsons Brinckerhoff. That compares with the 3 trillion to 4.2 trillion Btu of energy needed to build a managed lane mass transit system.

Another issue for the rail line is its impact on air pollution. Regional emissions would range from zero to 4 percent less with the rail system or an expanded bus service. That compares with zero to 4 percent higher for managed traffic lanes, according to Parsons Brinckerhoff.

Further details about the project's environmental impact are expected to be disclosed in a draft study scheduled for release this summer. Local environmental groups said they're reserving judgment on endorsing the transit project for now.

"There are certainly some benefits of rail, but the aesthetics, the route, the amount of energy used — all of that plays a role," said Henry Curtis, director of Life of the Land. "Therefore I don't know of any environmental groups that have looked at rail and said yes or no."