Former Energy Secretary Ernest Moniz came out in support of California’s plan to get all its electricity from “carbon-free” energy sources, but laid out two basic problems the state will have to deal with.

Moniz, who served under President Barack Obama, called a law working its way through the state Senate a “very, very big deal” during an interview with Axios.

California’s State Assembly passed legislation Tuesday night requiring the state to get 100 percent of its electricity from “carbon-free” energy sources by 2045, including solar, wind, hydropower and possibly nuclear.

The state Senate must pass the bill before it can head to Governor Jerry Brown’s desk. The bill seems poised to pass, which has the green energy industry and environmentalists excited.

“Here you have the fifth largest economy in the world saying we’re going to a carbon-free electricity sector in roughly 25 years,” Moniz told Axios.

However, Moniz laid out two major hurdles to getting to 100% “carbon-free” energy: battery storage and NIMBYism.

“Batteries clearly are making a tremendous impact already when talking about hours of storage,” Moniz told Axios’ Amy Harder. “But, what about when you need that backup for weeks or months. How are we going to handle that?”

What Moniz is talking about is making sure the grid maintains reliability. Solar and wind, which will make up the lion’s share of any effort to increase “carbon-free” energy, is intermittent, meaning dependent on sunshine and wind speeds.

Right now, California uses natural gas to maintain reliability and keep the electricity flowing as solar and wind power fluctuates. Nuclear power and hydroelectric dams are also used to maintain reliability.

If the state wants to phase out natural gas, it’s going to need to drastically scale up battery storage to offset the intermittency of solar and wind. (RELATED: Caliphornia? ‘Greenest’ US State Is Increasingly Reliant On OPEC’s Oil)

Moniz also mentioned NIMBYism. NIMBY stands for “not in my backyard,” typically referring to the anti-development mentality some communities can have, especially when it comes to big energy projects.

Property rights can mitigate some NIMBYism by getting buy-in from local landowners, but the sheer scale of what needs to be built in California to meet a “carbon-free” goal is on a scale not seen before.

“We’re talking, here, a deployment on an unparalleled scale,” Moniz said.

And he’s not exaggerating. Manhattan Institute senior fellow Robert Bryce ran the numbers and found the land use requirements were staggering.

Bryce found that “California would need 41.5 billion square meters, or about 16,023 square miles, of turbines” to get the wind power capacity needed to meet the state’s “carbon-free” goal.

“To put that into perspective, the land area of Los Angeles County is slightly more than 4,000 square miles — California would have to cover a land area roughly four times the size of L.A. County with nothing but the massive windmills,” Bryce noted.

For solar energy, even more land may be needed. Bryce wrote that “an all-renewable California would need more solar capacity in the state than currently exists on the entire planet” based on what experts estimate is required.

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