19th-century solar panels were made of selenium. Today, photovoltaic (PV) panels use thin wafers of silicon crystal. As photons strike them, they knock electrons loose and produce an electrical circuit. These subatomic particles are the only moving parts in a solar panel, which requires no fuel and produces clean energy.
Small-scale solar systems, typically sited on rooftops, accounted for roughly 30 percent of PV capacity installed worldwide in 2015. In Germany, a leader in solar, rooftops boast 1.5 million systems. In Bangladesh, population 157 million, more than 3.6 million home solar systems have been installed.
Rooftop solar is spreading as the cost of panels falls, driven by incentives to accelerate growth, economies of scale in manufacturing, and advances in PV technology. Innovative end-user financing, such as third-party ownership arrangements, have helped mainstream its use. Yet, costs associated with acquisition and installation can be half the cost of a rooftop system and have not seen the same dip.
In grid-connected areas, rooftop panels can put electricity production in the hands of households. In rural parts of low-income countries, they can leapfrog the need for large-scale, centralized power grids, and accelerate access to affordable, clean electricity—becoming a powerful tool for eliminating poverty.