Pretty stupid, but interesting:
In theory, the Tokyo area could import electricity from the south. But a historical rivalry between Tokyo and the city of Osaka led the two areas to develop grids using different frequencies — Osaka’s is 60 cycles and Tokyo’s is 50 cycles — so sharing is inefficient.

There are transfer stations, but they have limited capacity. And the hand-off is comparable to two railroads that use different gauge tracks and have to unload cargo from one train and reload it onto another at the place the tracks meet.
Short-term, energy efficiency will be the name of the game. Longer-term, it might just be best to integrate the grid.