TOKYO, Dec 22 (Reuters) - JERA, Japan's top power generator, said on Friday its new No.2 coal-fired power unit at its Yokosuka thermal power station, near Tokyo, has started commercial operation ahead of schedule to meet the peak heating demand during the winter season.

The move comes despite mounting criticism from climate activists and some investors over its continued use of power plants that are fired by the polluting fuel.

The launch of the 650-megawatts (MW) unit, initially slated for February next year, follows the commencement of the No.1 unit with 650-MW output capacity at Yokosuka in June this year.

Both units use an ultra-supercritical (USC) power generation system, the company said. The USC has higher efficiency than older systems in coal-fired plants, making it less polluting.

JERA, a joint venture between Tokyo Electric Power Company Holdings and Chubu Electric Power, said it will continue to steadily replace aging equipment with modern power plants to contribute to stable electricity supply and reduce carbon dioxide (CO2) emissions.

Japan has built a series of new coal-fired power plants over the past several years as it struggled to restart nuclear power stations which remain largely offline in the aftermath of the Fukushima disaster in 2011. (Reporting by Yuka Obayashi; Editing by Emelia Sithole-Matarise)