Informal Settlements (ISs) are a widespread phenomenon in cities in the global south. In Sub-Saharan Africa (SSA), about 56% of the urban population lives in ISs [1]. With an annual urban growth rate of 4% [2] paired with a lack of affordable, developed land, inadequate city planning, and inefficiently performing public governance [3,4], ISs will continue to exist and grow. In the absence of space for adequate housing, migrants settle in areas still available and previously avoided by citizens for environmental or health reasons. ISs have often been ignored by the country’s administration in order to prevent creating the appearance of legitimacy for these regions [4,5]. Illegal residents lack political power and do not benefit from political measures [6]. Occurring challenges within the settlements are being neglected or addressed reactively with a focus on resolving short-term risks [7,8], leading to limited improvements in the quality of life of its population. The image of residents has improved over the last decades, and numerous slum upgrading programs have been launched [9,10]. However, the fast-changing environment, their complex, convoluted structure and the illegal status of residents complicate the subsequent introduction of sustainable, long-term programs or an improvement of the infrastructure [11,12]. ISs apply to areas built without legal housing permits and outside the authority and administration system [13,14]. Figure 1 represents the typical view of an IS.

Due to economic vulnerability and the location of settlements in areas with challenging environmental conditions, residents are particularly affected by the consequences of climate change. Although the trend in SSA has steadily improved since 2013, the COVID-19 pandemic led to a renewed increase in the share of the population without access to electricity, which can be estimated at 600 million for SSA in 2021 [15]. Despite the fact that its electrification rate is generally higher in urban than in rural regions, more than 20% of the urban population still lacks an electricity supply, and the majority belong to ISs [16].

In ISs, the energy supply is often limited, unreliable, and expensive [12,17]. Many people living in these areas do not have access to legal grid electricity and furthermore rely on costly, traditional, and polluting energy sources such as kerosene, diesel, gas, and biomass. [11,18,19,20] Fossil fuels, such as (char-) coal or firewood, are often used for cooking, which accelerates environmental depletion and has a negative impact on health and the environment [21,22]. Some companies offer prepaid gas refills [23], but this form of energy is also fossil in nature. Lighting with candles or gasoline is still common, which can lead to fire outbreaks [24].

Access to electricity is usually measured by the ratio of to the grid-connected beings [25]. However, this yardstick does not reflect the complexity of reality, as the Multi-Tier Framework exemplarily confirms [26]. Aside from the fact that there are parts of the urban population that are not connected to the grid at all, other parts live unconnected but close to the grid or face an unreliable power supply. Dumitrescu et al. [27] introduced the end-user-market classification, which is being adapted within this context, given as an overview in Table 1 and subsequently described in greater detail.

Off-grid: Energy supply companies often charge a high amount to connect new customers to the grid. In many cases, ISs residents cannot afford these high upfront costs [28,29]. The poor geographical location of ISs (e.g., flooding) makes sustainable installation difficult, may increase maintenance, and leads to higher supply costs [30]. Rising energy prices increase the barriers for dwellers to able to meet the monthly charges [18]. Their illegal status, lack of tenure, and high residential turnover prevent long-term contracts with the utility company. Untransparent communication, lacking capacities to improve the infrastructure, and absent community involvement in ongoing projects further impedes the mutual trust between the utility company and residents [31,32]. For lighting, if electricity is not available, often candles or kerosene are used, which are economically and environmentally unsustainable sources [32,33].

Close-to-the-grid: The nature of ISs is often characterized by extremely narrow, sometimes impassable roads. This makes it difficult for electricity supply companies to build or maintain grid infrastructure [12]. Often, due to the building density and lack of accessibility, only residents on the main road of the settlement are supplied with legal electricity [11]. In Mozambique, the electrification of buildings that are not accessible by road is prohibited because, in the event of an accident caused by electricity, rescue vehicles cannot reach the houses [34].

Weak-on-grid: Improving electrification coverage and radius does not automatically result in universal access. A large number of slum dwellers report blackouts lasting hours, days, or even weeks and frequent voltage fluctuations, which can damage electrical equipment [11,12]. Outdated transmission- and distribution infrastructures additionally lead to reliability problems due to above-average losses. Natural events (e.g., flooding) can further damage the infrastructure. Intentional interference (e.g., theft) or mismanagement by utility companies can also contribute to unreliable supply [27]. At the same time, Grid Expansion must respond to the additional demand of the surplus consumers, whereas responsible institutions are often unable to organize new supply sources [18].

Illegal connections: Illegal electricity connections, often provided by cartels [35], are the consequence of those challenges. These supply systems are widespread in ISs, especially due to the resellers’ knowledge of the energy needs of the residents [36]. In the settlement of Mathare, Kenya, about 50% of electricity connections were informal between 2017 and 2019 [20,37]. Indirect connections can lead, due to a lack of electro-technical expertise, to several issues: Both health and safety are at risk from fire outbreaks or damaged electric appliances due to frequency oscillations and resulting blackouts. Furthermore, they endanger the reliability and security of the respective national power utility [19,36].

Clean, modern, renewable energy systems (RES) are not only mitigating climate change [38] but at the same time, are directly connected with benefits for the sector’s well-being, health, economic development, and education [12,39,40,41]. The transition from carbon-based to RE-based forms of energy reduces emissions, therefore smog pollution, and risks of fire outbreaks, which both are prominent in ISs. With access to electricity, public institutions and social services have a higher functionality and can be utilized to a greater extent. With the accompanying higher availability and usability of smartphones, residents of ISs have better access to information, online services, education, and communication [25]. The implementation and use of RES can further lead to job creation and revenue gain [39], which can help to address the proportionally high unemployment rate in ISs, which is vital for an improvement of living standards [25,42].

If ISs are not included in future energy scenarios, risks slowing down the transition to RES-supply and an increase in poverty of already marginalized groups occur [43]. Jaglin [44] considers the energy supply in cities to be a central task in the future due to demographic growth, city expansion, increase in energy demand, and current unreliable power provision. To support people facing the challenge of lacking or unreliable electricity supply in ISs, there is a need to improve its match between consumption and generation.

There are several ways to enhance the energy supply in ISs, which can address the above-mentioned energy challenges. The suitability of using different RES depends on a range of factors, such as the potential area of application and the local preconditions. The solutions considered in the framework of this article are Pico Solar and Solar Home Systems (SHS), Micro- and Mini-Grids, and a concept called Energy-Hubs. In the following, Pico Solar and SHS are merged into one category, hereinafter referred to as SHS. Micro- and Mini-Grids are also combined into one category, whereby within this article, the term refers to a classic Mini-Grid that supplies individual buildings, each with its own power connection. The Mini-Grid is defined as a 100% solar-powered island grid with the support of a battery energy storage system (BESS), while the use of diesel generators is not considered. The mentioned renewable energy-based solutions are being compared with the option of Grid-Extension, as displayed in Figure 2.

Grid-Extension means the extension and densification of the grid infrastructure. The Energy-Hub [45] includes all concepts that offer energy services for households and businesses at a central location, as implemented by, for example, a solar kiosk [46,47,48].