**5. Discussion of the Findings**

Investment in renewable energy technologies is essential to cut greenhouse gas emissions [86]. However, global CO2 emissions are increasing, between 2004 and 2019 the increase was 28.6% [87]. In 2020, the COVID-19 pandemic reduced energy demand and reduced global CO2 emissions by 8% compared to the previous year. This reduction resulted in a return to the CO2 emissions of a decade ago [88]. It is conjectured [88] that, as after previous crises, the rebound in emissions may nevertheless be greater than the decline, unless the wave of investment to restart the economy will be allocated more widely to cleaner energy infrastructure. Nevertheless, there is an urgen<sup>t</sup> need for policy action to curb the upward trend in CO2 emissions. On the positive side, there has been an increase in the use of energy from renewable sources—the only one at the time of the COVID-19 pandemic. It should also be noted that, despite the global increase in CO2 emissions, Europe has seen a fall in these emissions, thanks precisely to investment in solutions that promote renewable energy sources.

The transition to low-emission energy sources is occurring at different speeds in individual Member States of the Community. This is due to the divergent energy security interests of these countries and leads to dissonance in the energy union [89]. Achieving the objectives of the new EU energy strategy setting the goal of reaching 32% of the energy balance from RES in 2030 [90] requires reducing dependence on fossil fuels. The unbalanced perception of this issue and the different security priorities among the EU Member States result in a new west-east division of the Community, thus perpetuating the division that

has existed since the former Union of Soviet Socialist Republics (USSR) satellite countries joined the EU organization [91]. The own study shows that with regard to the household sector, a higher share of RES energy—in relative terms—in final energy consumption occurs in the so-called "poorer" part of Europe, while in countries with higher living standards [92] (Ireland, Luxembourg, UK, the Netherlands, Belgium, Sweden, Germany) RES use remains at a lower level. This may be due to the fact that improvements in the quality of life manifested by an increase in the area and standard of furnishings of dwellings and the possibility of obtaining an optimal indoor microclimate [93,94] result in significant energy consumption, which consequently diminishes the relative increase of RES in final energy consumption. In EU countries, the share of expenditure on energy consumed by households has an increasing trend [95]. Income growth is a strong determinant of increase in household energy expenditure [37,96,97]. Income is also a key determinant of investment spending on RES energy extraction. Thus, when considered as a whole, the reason for the smaller improvement in overall RES use is the higher living standards and changing lifestyles (more household appliances and larger dwellings) of today's consumers. Even though the share of RES energy is increasing, the overall increase in consumer energy use diminishes these achievements.

It is worth mentioning that the leaders in absolute terms in the use of RES by sector households are the French, Germans and Italians. According to own research, these countries also saw the highest growth in RES use by sector households between 2004 and 2019. It is noted that Western European countries, but also Scandinavian countries, have for many years been taking and supporting measures to increase the share of RES in total energy production. This includes tax incentives and educational measures as well as public support programs in subsidies for investments in renewable energy [98–100]. At the same time, the energy transition is more manageable in these countries because these economies are not based on fossil fuels (coal) [101]. The leadership role in the use of RES by Western European and Nordic countries is emphasized in many publications [60]. It is worth mentioning that Germany is the largest energy producer in the EU, i.e., 19.7% in total energy production in the whole European Union. Germany is followed by France, with an energy production of 17.6% and the United Kingdom with 10.6%. Italy 8.8%, Spain 8.7%, Poland 5.0% and Sweden 4.8% are also responsible for 3 4 of total energy production in the EU [102].

The self-analysis grouped the EU-28 countries into 6 groups according to the size and structure of household RES used. This classification indicates that the countries with the highest use of RES in households are the countries of Central, Eastern and Southern Europe. In these countries the dominant RES category is primary solid biofuels. It can be expected that in these countries, due to lower living standards [92], citizens cannot afford to invest in more expensive RES technologies. Deciding to use primary solid biofuels is a cheaper RES alternative than, for example, heat pump installations. Primary solid biofuels include, among others, wood pellets. In Poland, for example, the introduction of the "Clean Air" program (EU support) has widely promoted the replacement of old boilers using non-renewable solid fuels with boilers using biomass. Of all heat source applications, 1/5 requested subsidies for biomass boilers [103]. Some countries have even committed to increasing the use of wood pellets. Croatia, Slovenia and Slovakia by signing the Kyoto Protocol (The Kyoto Protocol) have committed to reduce GHG emissions precisely by promoting the use of wood biomass, primarily wood pellets [104].

In countries in the richer part of Europe, the use of primary solid biofuels in the RES use structure is usually at a lower level. In Western and Northern Europe (e.g., Ireland, The Netherlands, Finland, France) a relatively high use of heat pumps in households is observed. Heat pump technology is regarded as one of the environmentally friendly solutions for increasing energy efficiency and reducing harmful gas emissions into the atmosphere. As indicated, the use of heat pumps is economically beneficial in the Baltic region and the market share of these systems is increasing [105].

Solar thermal technology is also popular in richer European countries such as Ireland and Luxembourg. It is submitted that the interest in solar thermal energy may continue to grow due to the COVID-19 epidemic, which has changed the balance of energy consumption across countries. In particular, household energy consumption increased as people were encouraged to stay indoors [106]. This fact may lead to an increase in solar PV investments in residential buildings [107].

For climatic reasons, it is obvious that solar thermal technologies are most popular in households in the south of Europe (Cyprus, Malta, Greece). Cyprus, which formed a single-element cluster, records the highest use of RES in households. Cyprus is the world champion in terms of solar energy applications [108]. Already in the first decade of the 21st century, it was estimated that about 90% of residences in this country had a solar water heater installed. This record in thermal applications was mainly attributed to favorable weather conditions, a pioneering solar energy industry and a strong coordinated effort by all the concerned [109]. In Malta, on the other hand, the use of solar thermal energy, although dominant among household sources used, is generally in a smaller proportion of households. Solar water heaters in the first decade of the 21st century were present in several percent of all dwellings. It is surprising as the Maltese islands have the highest insolation in Europe [110]. The wider uptake of RES in Maltese households occurred in the second decade of the 21st century. As indicated [111], driven by the need to meet mandatory European Union (EU) renewable energy targets and facing the constraints of a limited territory, Malta was one of the first countries to rely almost exclusively on households to meet its clean energy targets. In 2009, a subsidy scheme was launched to encourage households to install photovoltaic systems on their own properties to feed into the energy network [111].

The results of our own research indicated that the leaders in EU-households respecting the RES share in final energy consumption (in relative terms) are Croatia, Slovenia and Latvia, among others. Croatia has the technical and economic potential of renewable energy sources necessary to achieve 100% RES in energy consumption [33]. The solar energy potential in Croatia far exceeds both existing and future, energy needs. The sunniest parts of Croatia, receive about 40% more solar energy than Central Europe and 60% more than Northern Europe. In winter, the continental part of Croatia receives twice as much solar energy as Northern Europe, with the central and southern coastal parts receiving 3–5 times more than Northern Europe, or twice as much as Central Europe [33]. Croatia is also rich in biomass and waste, hence the high use of solid biofuels. It is worth noting that in the late 1970s this country was one of the few regions in the world to initiate a solar energy program in response to the 1973 oil crisis [34,112].

From our own research we found that in Polish households, although there is a significant statistical increase in RES, the share of solid fossil fuels still remains at a relatively high level. Between 2004 and 2019, the increase in the amount of renewable energy used was at the level of the EU-28 average. As indicated [113], there is potential for development of RES technologies in Poland. Solar energy, wind power and solid biomass processing have the greatest chances for development.

Yang and Zhao [114] analyzing the financial aspect of fossil resource use for shorter periods found that fossil energy generally has lower financial costs compared to renewable energy but based on the conditions of sustainable development and then long-term projections, renewable energy is the only way to achieve sustainable living in the world. The energy system must provide energy services that are socially acceptable, economically sustainable and environmentally friendly [114].

In comparison, in countries neighboring Poland, i.e., Germany and Slovakia, the process of transition of households from fossil energy sources to renewable energy sources has been more favorable. Currently, household sector uses only a small percentage of fossil fuels in these countries. In general, solid fuels in Poland constitute the largest part of the energy mix on the EU scale, which significantly complicates the achievement of environmental goals, especially with the emphasis on decarbonization [84,115]. The transformation of the energy market in Poland depends on the financial situation of all energy producers (mainly coal companies in Poland) and power generators, whose activity is exposed to high financial risks [116,117]. The evolution of the Polish energy system is mainly influenced by the necessity to integrate energy markets in the EU. Despite the difficult conditions of the energy system in Poland, structural changes towards meeting the adopted environmental requirements are taking place. Therefore, the energy policy of the Polish country focuses on the energy supply security. In addition, competitive costs, minimal environmental impact and increase in energy efficiency are taken into account [58].

In conclusion, it can be stated, following B ˛ak and co-authors [101], that there are many factors influencing the disproportions between EU countries in the use of RES. Each EU Member State should look for the reasons why these disproportions become in order to answer the question how to improve its position in terms of RES use and how to change its policy to be more effective [101]. However, it is indispensable to include the household sector in the transition process from non-renewable to renewable energy sources.

It is impossible to compare the results presented in this paper with the results of similar studies because they have not been conducted so far. To date, taxonomic studies have been carried out on renewable energy in a broad sense, but not only on household consumption of renewable energy. The country classifications obtained in the studies quoted in the second part of the article do not coincide with the classification presented in this paper. It is not surprising that diverse cluster analysis results have been found, since they refer to a variety of aspects in the renewable energy sector and take into account multiple variables for the study. For this reason, the results presented in the paper contributes further to the issue of renewable energy sector in EU countries.
