Solar Photovoltaic (PV) applications are classified into three: solar lanterns, solar home systems (Figure 1), and solar mini-grids (Figure 2). Solar lantern is defined as small solar units of less than 10 Wp, while the solar home system ranges from 10 Wp to 150 Wp, and solar mini-grid is defined as an isolated grouping of electricity [4] . Solar mini-grid or solar grid combines multiple solar panels and associated equipment. The different classifications of solar PV offer the users the flexibility of choice depending on the applications.

There are some limitations to solar Photovoltaic (PV) applications, notwithstanding their general adoption. Solar PV needs an energy backup due to the intermittency of solar energy; thus, battery energy storage is applied. A feature common to renewable energy resources, especially solar resources, is light energy density, as compared to the energy from fossil fuel resources, implying larger solar energy systems are needed to produce limited electricity. The cost of installation of solar energy systems is high, especially the batteries. Solar PV limitations are summarised as storage issues and high price [1] , low-energy capacities [11] , intermittency of supply [3] and security of the installation, i.e., the location of the solar panels.

Notwithstanding its limitations, solar Photovoltaic (PV) has some advantages that accelerate its growth as an alternative electricity supply. It has low operational and maintenance costs [12] and promotes decentralisation of Nigeria’s electricity system [5] . A significant advantage of solar energy is its abundance and infinite nature. Solar energy is non-exhaustive and non-rivalrous. While the installation cost had been a cause of concern, Arent et al. [18] state that the cost of generating solar PV electricity has reduced considerably due to its increasing adoption. Still, Abdullahi et al. [7] believe that the reduction in the cost of solar PV electricity is due to technological breakthroughs. The gradual decrease in the cost of solar PV materials is due to factors such as economies of scale, technological innovations, and financial incentives. Another advantage of solar PV is its variety and flexibility of applications across solar lanterns, solar home systems and mini-grids.

Most studies present solar Photovoltaic (PV) as clean energy without a negative environmental impact and carbon emissions. It is of note that battery, a constituent of solar PV, poses a significant environmental challenge. The battery does not emit carbon during usage, but mining the raw material for its production causes substantial environmental degradation. In addition, the discharge of a used battery after its lifecycle is toxic to the environment. Lastly, storing arrays of batteries within a residential setting (Figure 1(b)) indirectly pollutes the household and leads to discomfort for solar PV users. Oghogho et al. [12] also argue that the by-products of manufacturing and utilising solar PV constitute environmental hazards.

Back-up generators (BUGs) have an installed capacity of 8 - 14 GW in Nigeria, serving approximately 80 per cent of Nigerians [19] . Okoye et al. [14] present data about Nigeria’s peak electricity generation and peak demand in 2013 and 2015. In comparing these data, it is evident that while the peak generation decreased during those years, the peak demand increased. To cover these shortfalls in electricity generation, people resolved to adopt BUGs (Figure 3). According to Adewuyi et al. [20] , despite the vast potential of solar energy, with an estimated generation capacity of around 43,000 MW, many Nigerians depend on inefficient and costly diesel and petrol combustion engine generators to meet energy. However, the broad adoption of BUGs has plenty of negative consequences, as Akin [21] reveals that using BUGs affects both the local and the global environment. Locally, the use of BUGs causes significant environmental and noise pollution, while globally, BUGs constitute a substantial contribution to global warming. In addition to the environmental and noise pollution, there is continuously a record of death across Nigeria resulting from inhaling the fumes

from the BUGs. Oguntoke and Adeyemi [22] suggest adopting a policy regulating BUGs, while Akin [21] recommends adopting solar PV to reduce the nation’s dependence on alternative energy solutions such as the BUGs. In terms of cost, the use of BUGs is more expensive compared to solar PV. While solar PV is costly at the point of installation, the zero operation and maintenance cost has given it an advantage over BUGs. Okoye et al. [14] compared the cost of electricity from BUGs and solar PV and discovered that it is more expensive to use BUGs than solar PV.

Nigeria receives about 4909.212 kWh of energy from the sun, equivalent to about 1.082 million tons of oil [8] . Solar radiation is fairly distributed in Nigeria, as displayed by the map of Nigeria in Figure 4 showing the global solar irradiation. Figure 4 indicates the spread of solar power across Nigeria, making everywhere suitable for solar PV applications nationwide. Muye [8] provides a broader range of solar irradiance in Nigeria. He says the solar energy potential in Nigeria ranges between 3.5 - 7.5 kWh/m²/day, with peak radiation in the North-Eastern part of the country.

Amid the abundance of solar energy resources in Nigeria, only a few states, such as Bauchi, Delta, Enugu, Lagos, Oyo, and Sokoto, have used solar PV to reduce their electricity burdens [26] . Emodi and Boo [27] reveal that some Northern states of the country, such as Kano and Katsina, have solar PV projects. Though solar PV deployment in the Northern part of the country did not occur until 1985 for rural electrification, it was followed by water pumping and

telecommunication in the subsequent years [15] . Yahaya and Sambo, cited in [15] , identified some of the solar PV projects in the North as the 7 kWp Gotomo village solar PV lighting in 1985 and the 7.2 kWp Kwakwalawa community solar PV electrification in 1993. The Nigerian telecommunications carrier, Nigeria Telecommunications (NITEL), recognised the positive impact of solar PV and incorporated it into the NITEL system in 1991; the Federal Ministry of Water Resources and Rural Development also installed solar PV water pumping machines across Nigeria in 1998 [15] . Despite these seemingly progressive developments of solar PV in Nigeria, Ohunakin et al. [13] believe that the deployment of solar technology in Nigeria is slow, corroborated by Bamisile et al. [11] with their opinion that solar PV is less utilised in Nigeria.

Solar PV has not been integrated into the national grid because Nigeria lacks utility-scale solar PV plants. While the Nigerian government targets a 2785 MW installed capacity of solar PV by 2020 [28] , Ozoegwu et al. [3] confirm that grid-connected solar PV does not currently exist in Nigeria. A report obtained from the Energy Commission of Nigeria [29] shows that in 2016, the Federal Government (FG) signed a Power Purchase Agreement (PPA) with local and international utility-scale developers expected to add 1200 MW solar PV capacity to the grid, as shown in Table 1. However, findings reveal that as of 2018, all the

projects have yet to commence due to bureaucratic delays from NBET [19] . How the 1200 MW solar capacity would be added to the grid is unclear because the Nigeria grid network is overloaded, resulting in frequent grid collapse.

The main driver of solar Photovoltaic (PV) in Nigeria is energy poverty and irregular electricity supply. The consumer’s choice of solar PV over the existing systems also stems from the need for reliable electricity. Despite Nigeria being a signatory to the Paris Agreement, climate change mitigation does not form part of solar PV development drivers in Nigeria. A 2018 report from the Intergovernmental Panel on Climate Change (IPCC) cited in [30] states that policymakers are not cognisant of the climate impact of the existing electricity sources. Most of the policies or strategies of the Nigerian government to promote the growth of solar PV are to bridge the gap in electricity generation and demand, not for climate change mitigation (Table 2).

A comprehensive list of barriers to the emergence of solar PV in Nigeria, as revealed by different authors and their recommendations, is compiled and presented in Table 3.

Energy policy combines intergovernmental responsibilities and market agreements, leading to complex regulatory and action frameworks [33] . The Federal Government of Nigeria plans to incorporate solar PV into the electricity mix so that solar PV electricity would constitute 1.26 per cent, 6.92 per cent, and 15.27 per cent of Nigeria’s electricity mix by 2015, 2020, and 2030, respectively [11] . Consequently, the following policies have been formulated: Public-Private Partnership (PPP) [7] , National Renewable Energy and Energy Efficiency Policy, and National Renewable Energy Action Plan [28] . Table 4 reveals other policies

extracted from the National Energy Policy [34] adopted to fast-track the emergence of solar PV in Nigeria. The slow emergence of solar PV in Nigeria is attributed to poor policy implementation Ozoegwu et al. [3] . Oghogho et al. [12] advised that the government should make a pragmatic effort to implement the adopted policies.

The research makes the following critical recommendations: the removal of the double subsidies on electricity and petroleum products; the introduction of import duty tax on BUGs; the implementation of government policies or strategies to facilitate the development of solar PV in Nigeria; the introduction of incentives for solar PV applications; the creation of awareness of the policies and incentives towards the promotion of solar PV applications; and broadening the applications of solar PV across various usages such as lanterns, home systems, street lighting, grid and off-grid, as revealed in Figure 5. The research also recommends that the Federal Government establish an agency, Solar Photovoltaics Development Commission (SPVDC), whose sole mandate would be to FastTrack the solar PV development and the deployment of its technologies in Nigeria.

Implementing the research recommendations offers the government of Nigeria a two-pronged solution to the challenges of energy accessibility and sustainability, alleviating energy poverty and mitigating climate change. In 2015, Nigeria joined other United Nations (UN) countries in France to sign the Paris Agreement to mitigate climate change by reducing global warming associated with carbon emissions, mainly from fossil fuel energy resources. Similarly, in the same year (2015), the United Nations (UN) Sustainable Development Goals (SDGs) advocated for sustainable energy for all by 2030 (Goal 7) and urged for climate action (Goal 13). Thus, the outcome of this research is relevant to two UN SDGs and the Paris Agreement.