Building on E3’s prior work for the California Energy Commission, this study examines which resources will be needed to maintain resource adequacy in a future California electricity system that is deeply decarbonized and heavily dependent on renewable energy and electric energy storage to meet California’s economy-wide 2050 greenhouse gas reduction goal. Whereas E3’s previous work identified resources California should build to meet GHG and renewable energy targets, this study takes an in-depth look at electric system reliability requirements – and specifically which resources are needed to maintain acceptable long-run reliability in a cost-effective manner. After examining resource adequacy through loss-of-load-probability (LOLP) modeling across thousands of simulated years using its RECAP model, E3 found that achieving economy-wide goals does not require full decarbonization of the electricity sector and that the least-cost electricity portfolio to meet 2050 economy-wide goals includes very large quantities of solar + storage and retains 17 GW to 35 GW of firm natural gas capacity for reliability.