TOKYO (Reuters) - Solar power generation contributed to about 10% of peak summer power supplies of Japan's nine major utilities, equivalent to more than 10 nuclear reactors, the Asahi newspaper reported on Thursday.

Though solar power accounts for about 2% of annual generation of all power sources, summer's favorable sunlight conditions increased power output, generating up to about 15 gigawatts of power in total in early August, the paper said.

Japan has been pouring billions of dollars in clean-energy investment after introducing a feed-in tariff (FIT) program in 2012, aiming to help the world's third-biggest economy shift away from its reliance on nuclear power after the March 2011 Fukushima disaster.

Asahi's survey showed that the ratio of solar power at peak hours was as low as Hokuriku Electric Power's 5.9% and as high as Kyushu Electric Power's 24.6%, depending on access to ample land with favorable sunlight conditions.

The installed capacity of solar power taking advantage of FIT scheme has reached more than 24 gigawatts at the end of April, government data showed, up from about 5 GW before the scheme started.

(Reporting by Osamu Tsukimori; Editing by Gopakumar Warrier)