In “The 100 Percent Clean Energy Act of 2018” the California legislature set a target of 100\% of California’s electricity generated from renewable and zero-carbon sources by 2045. The California Public Utility Commission (CPUC) and other state entities now have the task of planning to meet that target. The California Energy Commission (CEC) has sponsored multiple studies and sought public input on the pathways that they are exploring. A key result of that planning process is a Reference System Portfolio (RSP) based on existing and planned electricity generating capabilities, and modeled grid build out to meet the planned targets by 2045 at the lowest cost. Although this RSP has been discussed by the CEC in a public forum, to our knowledge, it has not been presented to the photovoltaic community. Here we document the CEC’s current RSP, with emphasis on understanding their expectations for build out of solar as well as the associated need for storage and curtailment.

Today resource adequacy is most often maintained by installing natural gas plants to meet the peak load. In California, the current risk of inadequate electricity supply is highest around sunset in late summer. In a zero-carbon grid, resource adequacy will increasingly require adequate stored energy throughout the entire year. Here we seek to develop an intuition about the times of the year when resource adequacy may be most challenged for a solar-dominant system. We use a simplified approach and show that the month of the biggest challenge occurs in winter and can shift by more than two months depending on the amount of solar and storage that are built.

Optimization models can be quite powerful in identifying a pathway to lowest cost zero-carbon energy systems. However, it is less obvious how to invert the models to calculate the target cost and duration of storage needed for that storage to be a significant solution. Storage is a dispatchable and flexible resource with the ability to perform many functions of grid support, further complicating the analysis. This paper complements existing papers by presenting an academic study of a simplified energy system, demonstrating a method for quantifying cost and duration targets for storage. The simplified analysis also helps to gain intuition about the synergistic relationship between storage and solar energy in a location like the state of California.