Almost 10 percent of the 2006 investments were in China, he said. India was the biggest net buyer of companies abroad in 2006, led by takeovers by Indian wind turbine maker Suzlon which is planning a European listing.
The report said worries about climate change, high oil prices averaging more than US$60 a barrel last year, efforts to break dependence on energy imports and government incentives to shift away from fossil fuels had spurred investment.
The report, prepared by UNEP with London-based New Energy Finance, said the wind sector won most investment with 38 percent of the total, ahead of biofuels on 26 percent and solar power on 16 percent.
Renewable energies are a key to fighting global warming, widely blamed on greenhouse gases from burning fossil fuels. A UN panel has projected that emissions will cause more floods, droughts, disease and rising oceans.
Of the total of US$100 billion, the report said US$71 billion included initial public offerings and spending on research and development of sustainable energy while mergers and acquisitions added almost US$30 billion.
UNEP noted that gains by many renewable energy stocks had far outpaced rises in world stock markets in recent months but toned down comparisons with Internet stocks which surged in the late 1990s before the dot-com collapse in 2001.
Unlike dot-com firms, the report said renewables were based more solidly on existing technology, that many companies were generating strong revenues and had regulatory support.
"Betting on companies that already have technologies is easier than betting on companies that are developing the technologies of the future," Eric Usher, head of UNEP’s Energy Finance Unit in Paris, told Reuters.
The report said that renewable energies accounted for 18 percent of investment in world power generation, or US$21.5 billion, compared with 2 percent of installed capacity.
The report also said the International Energy Agency, which advises rich countries, seemed conservative in forecasting that renewables would account for just 9 percent of power generation by 2030. UNEP scenarios ranged up to 23 percent of the total.
Story by Alister Doyle, Environment Correspondent, Reuters News Service