The new law gives California the most far-reaching clean energy goals of any USA state, along with Hawaii, which set a similar target in 2015 of 100 percent carbon-free electricity by 2045.
Brown, who signed the bill amid a huddle of environmental and legislative leaders, also issued an executive order pushing the state to reduce its net output of greenhouse gases into the atmosphere - including from the single largest source, cars and trucks - to zero by the same 2045 deadline. It sets a new policy to make 100 percent of California's energy resources come from zero-carbon sources by 2045.
Ever the state to lead the charge towards a more sustainable future, California has set an ambitious goal to rely entirely on renewable electricity by 2045.
Brown also signed an order that seeks to remove carbon from the air.
Brown's signing came days before he hosts a gathering of local, global and business leaders in San Francisco to highlight the urgency of addressing climate change. It also is less restrictive on technology, allowing any carbon-free resources to qualify including large hydroelectric dams and nuclear power.
The law requires utilities to source 60 percent of their power from renewable energy by the end of 2030, up from a prior goal of 50 percent.
While 32% of commercial energy sale previous year was purchased from renewable sources, one glaring problem California faces is having the overall energy supply matchup with overall demand for electricity.
"This bill and the executive order put California on a path to meet the goals of Paris and beyond".
The bill is in direct opposition to the current Trump administration efforts to roll back efforts to fight climate change.
"Every day, we see clear signs of a changed planet - from wildfires scorching the West, to record-high temperatures in the East, to rapidly changing oceans".
"Not here, not now", said Brown.
"California is committed to doing whatever is necessary to meet the existential threat of climate change", Mr. Brownsaid in his signing message.
"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. Meeting the 100 percent carbon-neutral goal in just 27 years gives California the most ambitious such target of any government in the world, the governor's office said.
The co-chairs of the San Francisco climate summit include former NY mayor and philanthropist Michael Bloomberg, UN climate chief Patricia Espinosa, and China's top climate official, Xie Zhenhua.
The clean energy bill, known as SB 100, passed the legislature last month.
But electricity accounts for only 16 percent of California's emissions. Kevin de Leon, a Los Angeles Democrat who is running for U.S. Senate against fellow Democratic U.S. Sen.