*2.3. California Grid Mix Composition in 2018*

In 2018 the total California domestic generation was 165 TWh. Figure 1 illustrates the California domestic grid mix composition in terms of total in-state electricity generated in the year 2018. Eleven % of the total in-state electricity was supplied by nuclear reactors, but as explained in Section 2.2.1, all of this is expected to be completely phased out by 2025.

Gas-fired electricity represented 39% of the total in-state generation, but that too is expected to decline due to aggressive California programs to minimize greenhouse gas (GHG) emissions. At the same time, though, it is also expected that gas-fired generation will continue to be a valuable technology for load following, and to compensate for the intermittency of wind and solar generation.

The remaining 50% of the total in-state electricity generation was supplied by RE technologies. Specifically, wind installations generated 10% of the total in-state electricity, while PV systems generated 16% thereof. As discussed in Section 3.1, the share of RE, and specifically PV, is expected to increase significantly over the coming years, with a concomitant surge in the required energy storage capacity.

Finally, electricity transmission was also included within the boundary of this assessment, albeit limited to the high voltage (HV) network. This was deemed an acceptable simplification, since the vast majority of the electricity generation plants comprising the grid mix at present and in the considered future scenario are centralised units which inject HV electricity into the grid. The HV transmission lines were modelled using the WECC-specific life-cycle inventory (LCI) information provided in the Ecoinvent database [38], and transmission losses were set at 6% as per historical data [61].

**Figure 1.** California domestic electricity generation mix—historical data for 2018 from CAISO. Total domestic generation was 165 TWh. SCGT = single cycle gas turbines; NGCC = natural gas combined cycles; PV = photovoltaics; CSP = concentrating solar power.
