The big goal is set, but the path to get there is still unknown.

California's 100 percent carbon-free electricity bill has nearly crossed the finish line.

The state legislature formally approved the decarbonization legislation Wednesday, which comes as the Trump administration attempts to revive coal power.

After Tuesday's more suspenseful Assembly vote, the Senate affirmed SB 100, authored by outgoing State Senator Kevin de León. If signed by Governor Jerry Brown, it will raise California’s renewable energy requirement to 60 percent by 2030 with interim targets, and give the state until 2045 to generate the rest of its electricity from carbon-free sources.

"It’s a real monumental time in California history, and maybe this will mark a turning point for the country," said Dan Jacobson, state director for Environment California and a supporter of the bill.

California will be the most populous and economically productive jurisdiction to commit to removing greenhouse gas emissions from its electrical grid. As de León told his Senate colleagues before Wednesday's vote, that's just the beginning. The state still has to tackle major greenhouse gas emissions from automobiles and buildings.

Where the world's fifth-largest economy leads, others tend to follow. Now California just has to figure out how to deliver on the promise of a carbon-free grid. That's going to be a lot more complex and interesting than just building more wind and solar.

Earning passage

If he signs it, the bill would burnish Governor Brown's legacy on climate change and clean energy ahead of a major summit on the subject in San Francisco next month.

It also gives de León a triumphant note to end his state Senate career on, as he challenges Senator Dianne Feinstein for her seat in November.

Despite the support of these and other powerful figures in California politics, SB 100 never had a surefire chance of success. Just last year, it failed to advance.

In that case, the unions representing utility workers withdrew support, citing concerns about distribution system jobs and the security of the grid.

This year, however, the International Brotherhood of Electrical Workers chose not to oppose the bill. Among other things, the major utilities and their associated unions had a new priority in addressing liability for the wildfires that tore through the state last year.

Though the bill hadn't changed, organized labor warmed up to it this year. A representative of the State Building and Construction Trades Council of California, with which IBEW is affiliated, spoke after the bill's author at the committee hearing July 3. Instead of airing concerns about the bill's impact on distribution grid jobs, he endorsed it as a job-creation opportunity, saying, "The industry is ready and waiting, the workforce is ready and waiting."

The 100 percent clean energy vision will force a departure from the electrical system's status quo, which introduces some risk, but it will also spur new construction of utility-scale facilities under project labor agreements, stimulating union jobs.

With labor on board, no major source of opposition remained, although several Republican legislators expressed concern about the bill's effect on California's electricity prices. Backers spent the last several days lining up the votes needed to assure passage ahead of the August 31 legislative deadline.

How it might work

In 2017, California got 29 percent of its power mix from renewables. Another 9 percent came from nuclear and 15 percent from large hydropower; both of those would count as carbon-free, although the state is closing Diablo Canyon, its last nuclear plant.

Natural gas provides 34 percent of California’s electricity.

The new energy directive has two major components: the near-term renewable portfolio standard increase and the longer-term carbon ban.

The first part will look like an extension of the current program of wind and solar power purchases, with gas plants balancing out the variable generation. Achieving 100 percent zero-carbon electricity requires a more profound shift: finding alternatives to natural gas for flexible capacity.

Furthermore, the text of the law stipulates that California must do this without causing additional greenhouse gas emissions elsewhere in the Western grid, meaning it cannot simply outsource fossil fuel combustion beyond state borders.

The challenge, then, is to not only derive most of the state's power from sunlight and wind, but to guarantee electricity when those resources are limited or unavailable.

"California has long been the home of big thinkers, innovators and change-makers, and while intermittency is an important issue, it can be addressed by a combination of solutions that include regional power transfers, energy storage, flexible demand and emerging technologies like wave, tidal and ocean thermal, which can be used to balance loads with supply,” said Ed Smeloff, director of grid integrations at solar advocacy group Vote Solar.

Energy storage technology currently tops the list of likely candidates for the grid-balancing role.

California pioneered large-scale lithium-ion battery deployments as a swift response to grid crises like the closing of the San Onofre nuclear plant and the gas shortage following the Aliso Canyon leak.

By now, California regulators and utilities have grown familiar with evaluating the technology for grid-scale applications. PG&E is building the world's largest batteries to replace gas-powered plants that are shutting down, in a preview of things to come.

The Assembly also passed SB 700, which extends the state's small-scale storage incentive for five years. That will boost deployment of batteries in homes and business, potentially lessening the burden on utility-scale facilities.

Questions remain about how much it would cost to build enough storage to make the zero-carbon system work. Recent battery plants have wielded four hours of energy duration, but technology to store energy for weeks or months remains a distant hope for the industry.

"Today's energy storage technology and the amount of transmission investment that would be required would be incredibly expensive to meet 100 percent compliance," said Wade Schauer, research director for Americas Power & Renewables Research at Wood Mackenzie. "Technology 20 years from now may make it less of a challenge, but the design of energy markets will need to be reinvented if all of the resources have variable costs of $0 per megawatt-hour."

Keeping options open

SB 100 originally called for 100 percent renewable energy, but switched to carbon-free energy in the course of its legislative evolution. That opens the door to other technologies, although it isn't clear which might come into play.

"The state is not trying to prescribe exactly what your energy is going to look like in 25 years — we don’t know what utilities are going to look like in 10 years," Jacobson said.

California previously opted to eliminate its existing nuclear resources. Diablo Canyon, which single-handedly supplies 9 percent of the state's electricity, must close by 2025. A bill passed last week would require that the plant's 2.2 gigawatts be replaced by clean generation, so that, after considerable effort, the state's share of clean electricity won't drop from the closure.

Gas generation that captures its emissions could qualify as a zero-carbon resource, but that might also face political opposition as California models a future of clean energy. Such technologies currently operate in a few places, but have not achieved widespread use.

Siting concerns and high costs have blocked new pumped hydro facilities, which are still the largest source of grid storage in the U.S.

With a 27-year time horizon, it's entirely possible that new technologies will mature by the time the zero-carbon rule kicks in. Just 27 years ago, wind and solar played negligible roles on the grid and large batteries were reserved for camcorders.

The flexibility of the law acts as insurance against locking out promising new tools.

"Personally, I don't think we will need the flexibility, because of the progress that solar, wind and energy storage are all making," wrote Daniel Kammen, founding director of the Renewable and Appropriate Energy Laboratory at UC Berkeley, in an email. "But in terms of building in the capacity for new technologies, and surprises, it is good policy to be diverse and inclusive in such long-range bills."

Framing the goal around ends rather than means improves its chances of success, MIT decarbonization researcher Jesse Jenkins wrote in an email.

"Study after study confirms that an inclusive mix of low-carbon resources is a more cost-effective and less risky route to decarbonize electricity than a more narrow focus on wind, solar and energy storage," he said.

Future legislatures could always revise the goals based on how the compliance process goes, although that would be a blow to California's reputation. The onus will be on regulators and the energy sector to show they can fulfill the mandate without ballooning costs for electricity.

For now, though, California has solidified its leadership role in building the future paradigm for clean energy and the grid. If it succeeds, others will learn from it. If it falls short, that expensive experiment will be instructive, too.