**5. Conclusions**

This analysis has shown that an energy transition in the California electricity sector hinging on the large-scale deployment of photovoltaic energy with lithium-ion battery energy storage (with a concomitant reduction in gas-fired electricity generation) would potentially be very effective at swiftly curbing GHG emissions, down to one half of the current level by 2030.

The non-renewable primary energy requirement per unit of electricity delivered could also be reduced by a factor of three, with benefits in terms of sustainability and positive implications in terms of domestic energy sovereignty.

Importantly, from the point of view of net energy delivery, contrary to previously voiced concerns, this analysis has also found that the overall energy return on energy investment (EROI) of a future electricity grid mix largely dependent on variable renewable energy plus storage does not have to suffer with respect to a current mix more heavily reliant on conventional thermal technologies such as nuclear and gas.

Additionally, the planned complete phasing out of nuclear energy in California does not appear to be detrimental to the future energy performance of the state's domestic grid, even when fully taking into account the mismatch between the hourly electricity demand and variable renewable energy resource availability profiles.

A degree of uncertainty remains on the future technological improvement trajectories for PV and battery technologies; however, all the aforementioned broadly positive results were produced when making rather conservative assumptions in both regard; further, a sensitivity analysis on future PV efficiencies and lifetimes has confirmed the robustness of the results.

A further source of uncertainty for the future is the possible change in electricity demand (both in terms of its hourly profile, and of the total year-end cumulative value) that could be brought about by a massive deployment of electric vehicles (EVs), with the associated requirement for battery charging. At the same time, though, a large EV fleet could also reduce the requirement for dedicated grid-level energy storage, by providing some of the required storage capacity through vehicle-to-grid (V2G) schemes. Accurately modelling the combined effects caused by these sector-wide changes was outside the scope of this paper but provides scope for future related research.

Future studies are also needed to address other potential environmental impacts in categories such as metal resource depletion and human and ecological toxicity. However, these types of impact are much harder to quantify, due to current uncertainties on emissions from mining activities [73], a range of methodological challenges, both in terms of characterization [74–77], and of the required but often delicate and difficult assumptions in terms of allocation [78].

**Supplementary Materials:** The following are available online at http://www.mdpi.com/1996-1073/13/15/3934/s1, Figures S1–S12: Complete projected hourly electricity generation and demand profiles for the entire year 2030, broken down by month. Table S1 and Figure S13: Sensitivity analysis on the % VRE curtailment, resulting from alternative nuclear and storage deployment hypotheses in 2030.

**Author Contributions:** Conceptualization, M.R.; methodology, M.R.; software, M.R., A.P. and E.L.; validation, M.R., E.L. and V.F.; formal analysis, M.R.; investigation, M.R. and A.P.; resources, M.R. and A.P.; data curation, M.R., E.L. and A.P.; writing—original draft preparation, M.R., A.P. and E.L.; writing—review and editing, M.R. and V.F.; visualization, M.R., A.P. and E.L.; supervision, M.R. and V.F. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported in part by the Faraday Institution [grant number FIR005].

**Conflicts of Interest:** The authors declare no conflict of interest.
