First of all EVs do not need that much power. We are talking something like 25% more electricity production for a country like Japan. Then Japan has rather a lot of onshore and even more offshore wind potential. Mountains are a problem, but hardly something which can not be overcome. Solar can easily be installed on roofs and mountains are even less of a problem.

Also really important to say it. Combustion engines in cars are massivly inefficent. So an EV is still better for the climate, even if run with coal electricity. The other factor is that Japans population is falling. So they will need less power over the long term.