*2.1. Renewable Energy (RE) Mini-Grids (MGs) in the Developing Context: Definitions, Trends, Challenges*

In the context of this article, a mini- or microgrid is understood as interconnected loads and distributed energy resources that are grouped and controlled as a single entity within a set area and which can operate interconnected to the national electrical grid or as a decentralised island solution [13]. The terms mini- and micro-grid are used interchangeably in this study as both terms are commonly used in the relevant literature [14]. MGs provide connections to a number of customers in villages or towns that are located far from the main electrical grid and which can be owned and operated by utilities, cooperatives, village electrification projects or private sector companies [15]. The size of these systems and number of household connections can range significantly but lies usually between five kWp up to two MWp of installed capacity and can either be a hybrid system combining two or more sources of energy, for example, diesel and solar PV or based on a single energy source such as hydropower or solar PV, normally in combination with a battery system for energy storage [16]. Due to their sizing and capacity, MGs are usually considered to be more suitable for productive uses of energy than solar home systems (SHS) which are usually sized between 10 Wp and 250 Wp installed capacity [17,18].

To date, the overall majority of RE mini-grid projects in developing countries are financed through a mix of private sector investment and donor-funded grants of 50, 70 or in some cases even 100% of the total initial investment [19,20]. This model is designed to mitigate some of the financial risks associated with the implementation and operation of MGs in rural areas and to attract private investment. In theory, the blending of gran<sup>t</sup> and private financing should help deliver quality energy services at an affordable cost for the consumer but the overall question has been raised [21,22], whether this is actually the case and whether MGs co-financed by grants provide 'cheaper electricity', trigger energy consumption in low-income areas and contribute to the overall financial and social sustainability of MGs.

Despite acknowledging the significance of decentralised RE MGs for enhancing access to clean energy in developing regions and the demonstrated positive impacts on rural livelihoods [23] in sub-Saharan Africa (SSA), the sustainable operation of these systems is challenging and largely remains an unresolved issue, especially if the systems are required to operate in a market environment that requires cost-reflective energy tariffs [24]. Despite a number of commonly reported issues such as technical faults [25,26], limited local capacity for maintenance and operation [27], inappropriate financing models or inadequate operational models that lead to limited affordability of energy and paymen<sup>t</sup> defaults [7,28–30] the availability of case studies presenting comprehensive financial, technical and operational data and their systematic review focusing on the sustainability of RE MGs is still limited [31]. However, comprehensive case studies of MGs can substantially enhance the understanding of their successful implementation and operation [32]. Hence, the need for qualitative research to understand the drivers of MG sustainability incorporating the demand and supply side have been widely formulated [33,34] and are embedded in the overall discussion of sustainability scenarios and parameters for MGs in rural developmental settings. While specific data revealing details of the technical design, investment including capital expenses (CAPEX), operating expenses (OPEX) and electricity tariff-models can potentially provide essential learning lessons to overcome barriers to the sustainable operation of decentralized renewable energy solutions [34] they are closely connected to the question of how sustainability is defined in that specific context which shall be discussed in the next section.
