Both wind and solar generation are about 10 times less in the lowest months versus the top 3 months. If California reaches 80% for renewables there will be massive amounts of surplus during the summer months and would need 9.6 million megawatt-hours of energy storage.
Achieving 100% would require 36.3 million. California should reach on 50% of its electricity from clean sources by 2020 and could pass a bill to legally require 100% by 2045. In January, they voted to close a nuclear plant which is a carbon-free source that provides 24% of the electricity.
The amount and cost of the storage will go over $350+ billion because of the wide variation in solar and wind power generation.
$350 billion would still be less than 17 over-priced pairs of Vogtle AP1000 nuclear reactors. The two 1.1 GW AP1000 reactors are coming in over budget at about $20 billion for both. Twelve pairs of AP1000 reactors would cost $240 billion even with bad cost overruns and generate over 200 TWh that California needs in electricity.
A study found meeting 80% of US electricity demand with wind and solar would require either a nationwide high-speed transmission system, which can balance renewable generation over hundreds of miles, or 12 hours of electricity storage for the whole system. At current prices, a battery storage system for 12 hours for the USA would cost more than $2.5 trillion.