Heating and cooling consume the most energy of all end uses, accounting for nearly half of global final energy consumption. Most of this is generated from fossil fuels. In 2019, fossil fuels and non-renewable electricity met more than 77% of heating and cooling demand (IRENA, IEA, REN21, 2020). The energy consumed for heating and cooling is thus a significant contributor to air pollution and carbon dioxide emissions: heating and cooling accounted for almost 40% of energy-related emissions in 2018, a share that has remained almost unchanged for the past decade, owing to the continued dominance of fossil fuels (IRENA, IEA, REN21, 2020).

Half of the energy consumed for heating and cooling is consumed in industrial processes, while another 46% is used in residential and commercial buildings – for space and water heating and, to a lesser extent, for cooking. The remainder is used in agriculture for greenhouse heating and for drying, soil heating and aquaculture (IRENA, IEA, REN21, 2020). Given that heating water accounts for about 18% of household energy use (US DOE, n.d.), on average, and that demand for hot water is growing with household incomes, the decarbonisation of heating and cooling in general, and water heating in particular is thus a key element of the on-going energy transition needed to limit the rise in global temperatures to well below 1.5°C (IRENA, IEA and REN21, 2018, 2020).

Solar thermal systems for water heating, or solar water heaters, represent a mature technology that has been successfully deployed in several developed and developing countries for more than 30 years. In countries like Barbados, Cyprus and Israel, 80 -90% of residential houses have domestic solar water heating systems on their roofs. The deployment of solar water heaters is particularly needed in countries that rely on fossil fuel imports to cover their heating needs, or where the use of electric boilers and heat pumps may strain the electricity system or be unaffordable to many households (ETSAP and IRENA, 2015). In South Africa, for instance, water heating with electricity and gas can account for 30-40% of the energy bill for a typical household (Hohne, Kusakana and Numbi, 2019)