California's most recent bill only states that electricity has to be zero-emissions, so theoretically the state might be able to adopt a full carbon-capture system on a natural gas plant, which is now being tested in Texas.
At a signing ceremony in the state capital Sacramento, Mr Brown vowed to meet the terms of the Paris agreement and to "continue down that path to transition our economy to zero carbon emissions". A bill Brown signed Monday sets a goal of phasing out fossil fuels from the state's electricity sector by 2045.
"There's no understating the importance of this measure", Brown said, moments before signing the two actions. Also on Monday, Brown issued an executive order establishing a new target to achieve carbon neutrality by the same year.
Later this week, Brown will host subnational governments, cities and businesses in San Francisco for the three-day Global Climate Action Summit.
The state body that regulates public utilities requires retail sellers of electricity to be working toward certain minimum requirements for the use of renewable energy sources. Yes, they do. So what we believe is that people are increasingly seeing the symptoms. "But it must be done", said Mr. Brown, a Democrat who leaves office after the November election.
The law sends a message that California supports the global Paris agreement to fight climate change, he said.
"It's impossible to overstate how significant it is for a state as large and influential as California to commit to 100 percent clean energy", Sierra Club Executive Director Michael Brune said Monday in a statement. After that, he says the state should emit net negative greenhouse gas emissions.
California follows the example set by Hawaii, which set the same 2045 goal in 2015.
Previously, California had mandated 50 percent renewable electricity by 2030.
Renewable energy experts have looked to batteries that can store solar energy generated in the afternoon as one solution, but the technology is not ready for wide-scale deployment.
"If we're going to have these first-in-the-nation laws, we want to see first-in-the-nation benefits", said Rob Lapsley, president of the California Business Roundtable. Kevin de León, D-Los Angeles.
Data from the U.S. Energy Information Administration shows California is ranked sixth among states in crude oil production and 15th in natural gas, though production of both has declined since the mid-1980s.
The new bill mandates that least 60 percent of electricity will have to come from renewable energy, especially solar and wind.
In 2017, California's zero-carbon sources - including nuclear, large hydroelectric and other renewable generation - accounted for more than 56 percent of total in-state generation of electricity.