## 1 Introduction  
To meet the challenging targets of climate change mitigation and ensuring affordable access to energy to present and future generations, it is imperative to develop innovative sustainable energy strategies addressing all types of energy demands [1; 2; 3]. Despite the progress experienced in recent years, the International Energy Agency estimates that still 900 million people do not have access to electricity, most of them living in rural areas [4]. The lack of reliable access to electricity is considered a limit on people's opportunities and quality of life. The role of energy as pivotal to sustainable development is now widely recognized by the international community, as evidenced by the Sustainable Development Goals (SDGs) that include \"access to affordable, reliable, sustainable, and modern energy for all\" by 2030 as a target in the SDG7. The literature emphasizes that electricity access in rural areas should always be accompanied and sustained by other enabling activities and services to contribute to greater education attainment, enhanced business opportunities, and a higher quality of life [5]. Rural areas are generally sparsely populated, isolated, and characterized by high illiteracy rates, lack of access to healthcare, and lack of a clean water supply, which lead to \"standards of living that almost universally lag far behind urban areas\" [6]. This situation is even more stressed by the limited progress in rural electrification due to the high cost of grid extension, which is not balanced by gains in local markets [7]. The complexity of the problem becomes even higher if we consider all the activities that involve thermal energy too. Indeed, electric energy accounts only for about 15% of global total final energy consumption, while much larger shares are associated with transport (29%) and heating (52%) needs [1]. Households account for the majority of energy consumed in rural areas--they require energy mainly for cooking, water heating, lighting, and space heating. According to [PERSON] [8], up to 80-100% of households' energy consumption is devoted to cooking and water heating that, in cold climates, indirectly can supply also space heating. Although in few emerging countries the situation in the last decades has improved and a general shift to modern energy has been achieved, the rural households often do not benefit from it since modern fuels and the related technologies may have higher capital and maintenance costs [7]. As a consequence, people that live in rural areas are often dependent on solid biomass fuels (SBF) for thermal energy needs, and the use of SBFs increase households' air pollution, bringing the levels of air quality and thermal comfort under the minimum standard conditions [9; 10]. What is more, the smart integration of multiple energy vectors and storage options--usually referred to as the \"Multi-Energy System\" or \"Smart Energy System\" approach [2; 3]--has been proven to allow for optimal energy system configurations that would remain unexplored within a single-sector domain [11]. Indeed, non-electric storage options are typically cheaper than their electric counterparts, and can be supplied by multiple energy vectors [11; 12]. For instance, thermal storage can be fed both by direct heat and by electricity (through heat pumps or resistances), thus ensuring an increased flexibility of the overall system and a significantly higher penetration of renewable sources, or reduction of use of a primary non-renewable energy supply [12; 13]. Adopting such a Multi-Energy System (MES) perspective might prove even more important in the context of off-grid areas, where both electric and thermal energy needs are typically satisfied by non-renewable fuels, associated with an unreliable and expensive supply-chain [14]. In such contexts, increasing the penetration of locally-available renewable sources would imply reducing the dependency on external fuel imports and the economic instability associated with price fluctuations.