Not every two countries use mutually incompatible mains systems; see for example [0] to see a real time map showing an electricity market with multiple nations

Not every two countries use mutually incompatible mains systems; see for example [0] to see a real time map showing an electricity market with multiple nations as participants. Note also, that your AC grids do not need to be compatible to transfer power between them, and even if they are, you might want to convert to and from DC in between - see [1] for examples of this. I have no idea why it is impossible for the Japanese connect their grids, but I am guessing the problem is not one of technology. Edit: Reading the Wikipedia article on the subject [2] it seems that the incompatible Japanese grids are indeed interconnected to a degree. It also seems that the electricity market in Japan being currently liberalized, which I take to mean that it has traditionally not been particularily open for competition. That might explain why nobody has been in a hurry to improve the system as a whole. [0] http://www.statnett.no/Kraftsystemet/ http://www.statnett.no/Kraftsystemet/ [1] https://en.wikipedia.org/wiki/High-voltage_direct_current https://en.wikipedia.org/wiki/High-voltage_direct_current [2] https://en.wikipedia.org/wiki/Electricity_sector_in_Japan https://en.wikipedia.org/wiki/Electricity_sector_in_Japan