Nippon Telegraph and Telephone is the world's largest rooftop solar power generation systems, installed over the roof of six buildings of the training center at Chofu City, Tokyo. The power generation capacity is 333kW and annual estimated power generation capacity is 380,000 kWh, which is equivalent to the amount used by 100 general households. It provides the power required for the 1000 PCs in the training center, as well as power for communication and power supply for life spot in the event of disaster. 

The system adopts low voltage grid connection in each building where solar cells are installed. This can reduces the construction costs and loss on wires. To utilize and save energy effectively, inverters are laying out in the shortest distance to the solar panels. Also it is designed to be miniaturized and noiseless.
On the other hand, making use of the cell's independence, it is also used as a life spot for 500 peoples at a disaster. (The NTT Central Training Center is one of the specified shelter in Tokyo.)