Worldwide energy demand amounts to around 13 terawatts of power. The amount of energy coming from renewable sources is a paltry 3.5% of the world's power needs. 228 GW of that power comes from solar energy.

Europe is currently the leader in solar or photovoltaic (PV) capacity, producing around 40% of the world's solar energy. In terms of growth, China and Japan are the global leaders, together comprising 51% of growth in PV installations in 2015.

Growth of PV installations over the past five years has been phenomenal, with a growth rate of between 29 and 42% every year. Projections of future growth are equally impressive, ranging from around 400 to nearly 700 GW of PV capacity in 2020.

Projections have historically underestimated estimate actual growth, however. The prices of PV energy continues to fall (see related question here) and projection methodologies may in some cases fail to capture factors such as transformative technologies that may further drive PV growth.

How much global solar photovoltaic electricity-generating capacity, in gigawatts, will be in operation by 2020?

The definitive source for question resolution will be the annual "Trends" report from the International Energy Agency's Photovoltaic Power Systems Programme. Because the report covers trends up to the year previous, resolution will be dependent on the report to be issued in late 2021. If this report should cease publication or substantially change its methodology, question resolves as ambiguous.