FRANKFURT - Revving up European power transmission networks to transport 90 percent of renewable energy by the year 2050 could be achieved at affordable sums, pressure group Greenpeace said in a study published on Friday.

European policymakers dream of getting away from fossil fuels but even if these were replaced with wind or solar generation systems, sceptics say the bloc's decades-old grid systems would effectively hamper shipping the volatile power.

Greenpeace said the cost of strenghtening cross-border lines and building new interconnections to create so-called smart or supergrids would be small if it was spread over 40 years and split between hundreds of million of Europeans.

All together, the proposal would cost around 209 billion euros ($310.9 billion), it said in a press release issued to accompany the report's unveiling in Berlin.

This would increase the costs of every kilowatt hour by 0.15 cents over 40 years which means for a European household less than five euros a year or 40 cents a month, it said.

Apart from the cost of preparing grids for new tasks to better manage erratic supplies, there is also concern that over reliance on wind or solar could leave consumers short of power when the wind does not blow or the sun does not shine.

The Greenpeace study compared 30 years of weather data with European annual demand curves and concluded that there is only a 0.4 percent -- or 12 hours a year -- chance that high demand correlates with low solar and wind generation.

Apart from wind and solar, it also mentioned chances to exploit geothermal and ocean energy, and biomass.

We just need smart grids to put it all together and effectively 'keep the lights on', said Greenpeace.

The 209 billion sum was broken down into 100 billion for 11 new connections inside Europe, 90 billion euros for new lines to capture Sahara desert solar power, 16 billion for upgrades of direct-current high voltage lines between European countries and 3 billion for alternating-current ones. (Reporting by Vera Eckert; editing by James Jukwey)