Japan's power generation mix is shifting away from thermal plants, in line with the global trend toward renewable sources. But the transition is also raising concern about a possible energy crunch.

Energy officials say Japan's thermal-power generating capacity has decreased by about 16 million kilowatts in the last five years as more plants were shut down or idled. That is equivalent to the electricity needed to supply 5.4 million households.

The profitability of generating electricity by burning coal, oil or natural gas is decreasing as solar power becomes more widespread. Attracting investment for thermal power plants that emit greenhouse gases has been difficult.

Takeuchi Sumiko, an energy policy expert at the International Environment and Economy Institute said the government should have implemented policies to maintain thermal plants that can provide power when solar and other renewable energy sources cannot generate enough.

The government is planning to ask the public and businesses to save as much electricity as possible this summer in anticipation of a power crunch. It will be the first time in seven years for the government to issue such a request.