* Losing nuclear could knock back decarbonisation 15 years
* Wave of nuclear decommissioning expected from 2020
* High nuclear load factor makes it harder to replace
* Choice for renewables requires grid, storage investment
ESTORIL, Portugal, June 21 (Reuters) - Decommissioning nuclear plants in Europe and North America from 2020 threatens global plans to cut carbon emissions unless governments build new nuclear plants or expand the use of renewables, a top International Energy Agency official said.
Nuclear is now the largest low-carbon power source in Europe and the United States, about three times bigger than wind and solar combined, according to IEA data. But most reactors were built in the 1970s and early 80s, and will reach the end of their life around 2020.
With the average nuclear plant running for 8,000 hours a year versus 1,500-2,000 hours for a solar plant, governments must expand renewable investments to replace old nuclear plants if they are to meet decarbonisation targets, IEA Chief Economist Laszlo Varro told Reuters.
"The ageing of the nuclear fleet is a considerable challenge for energy security and decarbonisation objectives," he said on the sidelines of the Eurelectric utilities conference in Portugal.
Renewables have grown rapidly in the past decade but about 20 percent of new low-carbon capacity has been lost from the decommissioning of nuclear plants in the same period, he said.
"This is just a taste of thing to come," Varro said.
Russia and India were building new plants, while China was bringing a new plant online every quarter, Varro said.
However, he said future projects in Japan were uncertain after the 2011 Fukushima disaster, while there was less appetite for new nuclear projects in Europe and the United States.
Financing new nuclear plants has become more challenging, partly because several new builds were running over budget, while in the United States nuclear has been struggling to compete against plants run on cheap shale gas.
Governments who chose the renewables route would have to consider upgrading power grids and invest in power storage to offset the variable nature of renewable generation, while those choosing nuclear would need to offer financial support as Britain has done for its plans, Varro said.
Wind and solar generation was expanding rapidly, but the pace needed to increase to meet climate stabilisation goals. "At the moment it is not quickly enough," he said.
The biggest challenge was in Europe and the United States, where nuclear energy capacity was steady or falling, he said.
"If we do not keep nuclear in the energy mix and do not accelerate wind and solar deployment, the loss of nuclear capacity will knock us back by 15 to 20 years. We do not have that much time to lose," he said.
(Reporting by Geert De Clercq; Editing by Edmund Blair)