**7. Conclusions**

The purpose of the presented analysis was to assess the changes that have occurred in the use of RES in the production of electricity in the EU and UK, and the main research period was set as 2005–2019. As different countries have different levels of use of renewable energy sources (RESs) in electricity production (GEP), in our main analyses, we focused only on the electricity generated from RES (*GEP\_RES*) and its amount generated from each RES. It is this approach that distinguishes the presented study from others presented in the literature that focus primarily on GEP-related analyses. In the presented study, we analyzed the shares of seven types of different sources (hydro, wind, solar, biofuels, biogases, renewable municipal waste, and others) in the production of *GEP\_RES*. The main research methods are the Gini concentration coefficient and the k-means algorithm.

The analysis shows that the Gini coefficients decreased for almost all countries in the period 2005–2019. This means that the concentration of renewable sources used for electricity production has decreased significantly across the EU. As indicated in the discussion, one of the main drivers of change in this respect (increased use of RES) has been the EU energy policy targets and national energy policies to adjust national energy sectors to these targets. This inclines us to accept Hypothesis (1). This phenomenon is in line with the recommendations of the European Commission regarding the diversification of energy sources, which is to support the energy security. It is worth noting that, while in 2005, in most countries, the predominant source of renewable electricity (with a share of over 80% in *GEP\_RES*) was hydroelectric power plants, in 2019, a significant increase in the importance of other sources occurred.

The level of electricity production in hydroelectric power plants did not increase significantly, but with the simultaneous significant increase in the production of renewable electricity, the share of this source decreased. Hydropower production is still of grea<sup>t</sup> importance and is the main renewable source for electricity production in many European countries (with a share of more than 50% in *GEP\_RES*). However, since this method of obtaining energy requires specific geographical conditions and the fact that it is not completely neutral to the environment, its development focuses primarily on more efficient use of already existing facilities, in particular those large ones. Wind energy is gaining importance. In 2019 in the EU-27, the share of this type of source in the production of *GEP\_RES* was already over 36% and was greater than the share of hydropower. This source is recognized as the most effective among renewable sources in the production of electricity. It is also a resource promoted by the EU bodies. In particular, the emphasis is on the development of offshore wind energy, which is included in the directive [12].

The significant shares of the abovementioned sources (hydro and wind) in the production of *GEP\_RES* in the vast majority of EU countries make those two the most important sources of renewable electricity. The remaining sources are usually treated as complementary. Among those sources of renewable electricity, solar energy, in particular photovoltaic energy, is important in the EU scale. It plays a key role in most of the countries of Southern Europe that is related to the level of insolation [119]. This technology, considered the second most effective renewable source of electricity after wind energy, is also developed and promoted in countries with less favorable climatic conditions for it: the Czech Republic, Germany, the Netherlands, and Belgium. The ecological aspects add to the importance of the solar technology and influence a support from governments of many countries (e.g., Germany and the Czech Republic). In northern countries, biomass— particularly biofuels— plays an important role as a complementary resource to the renewable energy mix.

Two sources that were categorized in this paper as "other" are of importance in only two countries: energy from the geothermal source produced in Italy and tide wave ocean used in France. As the first technology increased in importance (both in Italy and to small extent in other countries), the shares of tide wave ocean technology began to lose their importance. In the analyzed period, this source was not used to a greater extent.

The performed cluster analysis shows that, firstly, there are EU countries with a very individual structure of using the renewable sources, such as Malta (the only country where solar energy is used to a higher extent) or Estonia (a leading country in the use of biofuels). Secondly, there are countries where the use of renewable sources is highly diversified. An example is the Czech Republic, where as many as four sources had shares in *GEP\_RES* at the level of more than 10%. Thirdly, the vast majority of countries has been assigned to multi-element clusters, indicating that they have similar structures of consumption of renewable sources in *GEP\_RES*. Importantly, the clusters constitute countries often not closely located geographically to each other. As it turns out, geographical factors are not the only determinants of the amount of energy consumed, or the sources used for its production. The key factors here are the national energy policies and strategies that formulate the national goals and publicly supported technologies. Therefore, the presented study can be used as a basis for comparisons of impacts of national policies on the promotion and use of renewable sources for electricity production.

The presented study clearly shows that all EU countries implement the assumptions of the energy policy regarding both increasing the share of renewable sources in energy production—in particular, electricity—and increasing the diversity of these sources. The results indicate a vast diversity within the EU countries in terms of the use of the renewable sources for the production of electricity. This shows individual country-oriented approaches to implement the energy transformation towards low-carbon economies. The EU's support and restrictive measures (e.g., the already mentioned EU ETS) are of considerable importance for this transformation. Currently, the main source of renewable energy in the EU is wind energy, which is increasingly used by most EU countries, and the offshore renewable energy strategy (European Commission, 2020) is guiding the development of this type of energy. Maintaining the pace of the energy transformation allows the achievement of the goals set for 2050 in the Clean Energy for all Europeans document [11].

**Author Contributions:** Conceptualization, A.M.-J., D.Z.-S., U.A.-K. and M.H.-K.; methodology, A.M.- ˙ J.; validation A.M.-J. and U.A.-K.; formal analysis, A.M.-J., D.Z.-S. and U.A.-K.; investigation, A.M.-J. ˙ and D.Z.-S.; resources, D. ˙ Z.-S. and A.M.-J.; data curation, A.M.-J. and U.A.-K.; writing—original draft ˙ preparation, A.M.-J. and D.Z.-S.; writing—review and editing, D. ˙ Z.-S., U.A.-K., A.M.-J. and M.H.-K.; ˙ visualization, D.Z.-S.; supervision, A.M.-J., D. ˙ Z.-S., U.A.-K. and M.H.-K.; project administration, ˙ D.Z.-S. All authors have read and agreed to the published version of the manuscript. ˙

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

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