Renewable electricity generation is expected to double or triple in the next six years and outpace natural gas and nuclear power as a global source for electricity as early as 2016, according to a report by the International Energy Agency (IEA). If that happens, renewables will be second only to coal for electricity generation.

“Globally, renewable generation is estimated to rise to 25% of gross power generation in 2018, up from 20% in 2011 and 19% in 2006,” the IEA reports. That growth is being driven mostly by the expansion of wind and solar photovoltaics (PV) generation.

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As hyperbolic as that might sound, it’s probably an understatement, say some energy watchers.

“It’s a remarkably bullish outlook compared to most forecasts,” commented greentechmedia energy analyst Chris Nelder in a thorough post about the new report. “It’s particularly remarkable for the IEA, whose conservative outlook on renewables has historically lagged behind reality.”

Nelder suspects the IEA is, in fact, underestimating again and that the competitiveness of renewables depends a lot on where the wind or solar generation is happening and on the power mix in the grid in those places.

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The IEA reports, for example, that some onshore wind is competitive with new gas-fired plants in Brazil, Chile, and Mexico. But it’s not competitive in the United States, because natural gas there is very cheap at the moment. If U.S. natural gas prices were back up where they were a few years ago, wind would be cheaper in the states, Nelder explains.

“In Australia, wind is competitive with new coal- and gas-fired plants with carbon pricing, but only the best wind sites can compete without it,” Nelder writes. “In oil-exporting countries of the Middle East, solar PV is cheaper than burning oil for power generation, because the value of exported oil is so much higher. And so on.”

As for who the IEA is, they are an independent agency established in 1974 to promote energy security for its 28 member countries by responding to disruptions in oil supplies as well as to provide research on affordable clean energy.

IMAGES: A wind turbine stands on a field near Pulheim, Germany, 17 June 2013. (Federico Gambarini/Corbis)

Past and future predicted renewable energy electricity generation worldwide. The abbreviation OECD stands for countries belonging to the Organisation for Economic Co-operation and Development. Graphic courtesy of IEA.