## 3 Methodology and Application to a Case Study
### Main Socio-Economics Impacts  
In part C of the interview, experts were openly asked about the expected social and economic impacts of PtG deployment. The most mentioned impacts were increasing investment, employment generation, and mitigation of the environmental impact. Next, an approach is proposed to estimate the effect of PtG on the determinants of sustainability, namely, the estimation of the CAPEX of a generic installation, employment factors, annual production of synthetic natural gas (SNG), and annual CO\({}_{2}\) consumed. Because of the demonstration stage of PtG technology, the socio-economic impact of PtG in Spain was evaluated in scenarios that consider 2030 and 2050 as reference years for consistency with European decarbonization plans. Investment required, employment generated, and CO\({}_{2}\) captured were estimated considering a starting point of 35 MW installed up to 2025 and growing rates of an addition 250 MW installed per year for the period 2025-2030 and 400 MW per year for the period 2030-2050. This is a conservative assumption. Several studies (for example, of the aim of reducing the UK's greenhouse gas emissions to zero by 2050), evaluate this growing rate as up to 567 MW of electrolysis per annum for 30 years [77]. Several prospective reports, such as that of the U.S Department of Energy [78], consider that a suitable size for distributed electrolysis production is 1.5 t of hydrogen per day. The size of the unit plant was selected taking into account the annual operating hours needed to produce 1.5 t/day from the electricity generated in a generic PV plant on a yearly basis. Because H\({}_{2}\) production varies throughout the year, in addition to throughout the day, depending on the available radiation, a H\({}_{2}\) buffer is required. Considering that 100% of the available H\({}_{2}\) is finally used to reduce economic investment on equipment, we selected a small methanation reactor that provides twice the operating hours per year. The necessity of compression and storage systems for SNG was also considered. Accordingly, the PtG basic unit in this paper consists of a PV installation of 3.5 MW with 2500 h of utilization per year, a 2.8 MW electrolyzer (3125 h of utilization per year) with 70% efficiency, and a 1.4 MW methanizer (6250 h of operation per year) with an optimistic 100% conversion efficiency. Figure 1: Aggregated rating in typologies of barriers and incentives. This basic installation produces 510,117 Nm\({}^{3}\) of synthetic natural gas (SNG) and captures 1009 tons of CO\({}_{2}\) per year. To estimate the investment cost, the reported installation costs for every installed PtG system were considered, as summarized in Table 4. The data for alkaline and membrane electrolysis, and for chemical CO\({}_{2}\)-methanation, was projected using exponential approximation over average annual values by [PERSON] et al. (2019) [12] to include the expected decrease for the 2030 and 2050 horizons. We used a benchmark installation cost for a PV system of 1000 \(\upcels\)/kW, with a moderate reduction due to the degree of maturity that this technology has reached. Because no preference regarding the type of electrolyzer is currently shown in the reported installations [12; 25], a generic electrolyzer was considered with a unit cost equal to the average values for alkaline and PEM electrolyzers in Table 4.  
To estimate the capital expenditure of a PtG unit, 25% of the electrolyzer cost and 40% of the methanizer cost were added to consider batteries and gas storage, and compression, among other auxiliary systems. It should be noted that the estimation of the investment costs excludes the CAPEX of the CO\({}_{2}\) capture technology because CO\({}_{2}\) is considered as a reactant externally provided. Based on the unit costs summarized in Table 4, the investment cost for a PtG basic unit would result 7860 \(\upcels\) for the year 2025.