Several mornings a week, I leave my home in sunny Oakland, board a Bay Area Rapid Transit train and shoot underwater to work at Greentech Media's San Francisco bureau. In time, that commute to write about clean energy will be powered by clean energy as well.

The BART board of directors just passed an electrical portfolio policy that requires half of the organization's power to come from renewables by 2025, and 100 percent by 2045. That has broader implications for the region, because the electric train system consumes roughly 400,000 megawatt-hours annually -- equivalent to a small city like Alameda. And, due to a unique legislative carve-out, BART will procure this power itself.

"We're doing this to advance clean energy, but we're also doing this because we think it is cost effective," said Sustainability Director Holly Gordon, who previously led legislative and regulatory affairs at Sunrun. "We feel as though we can purchase clean energy while maintaining low and stable costs for the district as well."

BART already cuts area carbon emissions by lowering the number of single occupant automobiles sitting in traffic on the Bay Bridge each morning and evening. But the all-electric trains need considerable juice to run.

The board's decision, then, models a key strategic pathway for jurisdictions worried about climate change: electrify crucial activities, like transportation, then decarbonize the electricity.

There's a different model here, too: the transit agency as electric utility.

This will be the first use of a 2015 California law that gave BART in particular the freedom to choose renewable power suppliers on the wholesale market. Before that, the district could procure preference power from a federal power marketing agency or buy from a municipal utility.

Pacific Gas and Electric handles delivery, but BART has pulled together a power mix it says is 78 percent cleaner in terms of carbon emissions and 18 percent cheaper than what an equivalent customer gets from the utility.

The new energy policy will put BART ahead of California's renewable portfolio standard, which mandates 50 percent renewable power by 2030. Legislators have considered expanding that to 100 percent by 2045.

To achieve those targets, BART will release a PPA RFP next week. This is not a virtual PPA, Gordon noted -- companies will have to actually bid into the California transmission network.

The agency is also building onsite solar generation with SolarCity: two 1-megawatt plants are under construction at different stations; a half-megawatt plant already exists. With an 80-megawatt peak, though, in-house generation will only serve a fraction of demand.

This approach to sourcing power for transportation hearkens back to the earliest days of the grid, when trolley companies built and operated their own power plants specifically to run their vehicles. Incidentally, the first public electric grid opened up at Fourth and Market Streets in San Francisco, just a block from BART's Powell Street Station.

BART appears to be the first electrified public transit system to commit to 100 percent renewable power. Nearby Caltrain pledged to use 100 percent renewable electricity, but its trains still run on diesel.

As more cities buckle down on greenhouse gas commitments, the transit agency/renewables buyer could become a prominent class of commercial clean power customer.