Researchers with the Pew Charitable Trusts calculate that China invested $34.6bn (£23.2bn) in clean energy over the year, almost double the US figure.
The UK emerges in third place among G20 nations, followed by Spain and Brazil.
The most spectacular growth has come in South Korea, which saw installed capacity rise by 250% in five years.
Globally, investment has more than doubled in the last five years, Pew finds, with the recent economic turmoil generating only a slight dip.
The US still holds a marginal lead in the total amount of installed capacity, but will be overtaken by China during the course of this year if existing trends continue.