Japan opened several solar energy parks on Sunday as a new law came into force requiring companies to purchase renewable energy at a fixed price in a push for alternatives to nuclear power.

The openings come on the same day engineers began refiring an atomic reactor, despite growing public protests in the aftermath of meltdowns at Fukushima, ending nearly two months in which Japan was nuclear-free.

A new solar centre opened in Kyoto in western Japan, while various municipalities also started up installations able to provide energy for hundreds of thousands of households.

Japanese telecommunications Softbank chief Masayoshi Son, opposed to nuclear energy since a powerful earthquake and tsunami last year that crippled reactor cooling systems, said it had plans for 11 solar or windpower centres in Japan.

The push to invest in renewable energy resources is a mark of Japan's search for alternatives to nuclear power, as 49 reactors out of 50 in the country have been shut down for safety checks and amid growing public protests.

The new law that took effect on Sunday requiring power companies to purchase all renewable energy at a fixed tariff is aimed at encouraging firms to pursue sustainable initiatives.

The government estimates the power provided by renewable energy this year in Japan will attain 2,500 megawatts, the equivalent of two medium-sized nuclear reactors.