*3.2. Description of Calculation Avoided CO2eq Emission Method*

The avoided emission of CO2eq was calculated by the use of the life cycle assessment (LCA) method applied for the standard PV system supported by the "My Electricity" program. The general framework of LCA included in Intergovernmental Standardisation Organisation standards [56] was used to calculate CO2eq released during the lifetime of PV systems, using the cradle-to-grave approach [57]. The aim of the LCA analysis was to compare the emissions from the PV systems to the conventional energy sources dominating the Polish energy market, in particular hard coal, lignite, natural gas, and Polish energy mix, constituting the kinds of energy replaced by photovoltaics, as presented in relevant literature studies [58,59].

System boundaries of the PV systems analysed in the study included the production of 5.72 kWp installation (average power resulting from "My Electricity" program data) together with PV modules (including their market shares) and balance of system (BOS), transportation processes based on market reports including those about local producers and imported parts and servicing (washing, replacement of parts) [60].

The basic assumptions for LCA research were adopted from methodological guidelines and included service life of PV systems equal to 30 years, with partial replacement of inverter, while some elements of BOS such as metal construction for panels had a 60-year lifespan [57,61]. Life cycle inventory was based on the Ecoinvent database and leading producers' data, including the efficiency of monocrystalline panels equal to 20.5% and for polycrystalline panels equal to 17.2%. Adaptation to local conditions was based on "My Electricity" program summary data, region-specific energy yield estimates with an included decrease in panel efficiency described in paragraph 3, and statistical reports on the PV market in Poland, which enabled estimation of transportation distances and kinds of PV panels mounted in 2019 and 2020. According to the market reports, the average share of monocrystalline panels rose from 79.5% in 2019 to 97.65% in 2020, while polycrystalline technology recorded the decrease in share from 20.5% to 2.35% [60,62,63], which was included in the study in accordance to the number of installations built in the analysed years.

The method of CO2eq calculations was Global Warming Potential 2013 (IPCC GWP 100a), enabling calculation of climate change potential on the basis of 204 characterisation factors for specific emissions to air. The life cycle model was built in SimaPro v.8.1 software by PRE Consultants, Amersfoort, The Netherlands, with included Ecoinvent 3.0 database by Ecoinvent Association, Zurich, Switzerland. Sensitivity analysis by the Monte Carlo method was performed with 1000 runs and a confidence interval of 95%. The emissions from energy generation processes were updated to the levels of 2019 and 2020 by the use of modified Ecoinvent unit processes [21,64].

Absolute GHG emission avoidance was calculated in previously mentioned four basic scenarios of energy sources replaced by PV electricity according to Equation adopted by [65].

$$
\Delta GHG = \sum\_{y=1}^{30} \left( GHG\_y^{Rcpl} - GHG\_y^{PV} \right) \tag{8}
$$

where

Δ*GHG* Avoided emissions of greenhouse gases, Mg CO2eq;

*GHGRepl <sup>y</sup>* GHG emissions of replaced energy source in successive years, Mg CO2eq;

*GHGPV <sup>y</sup>* GHG emissions of PV systems built in the My Electricity program in successive years, Mg CO2eq.
