The ability of power companies in western Japan to sell excess power to Tepco is hampered because the country is divided by grids that use different frequencies. Companies including Tepco and Tohoku Electric Power Co. transmit power to the eastern grid at a frequency of 50 hertz (cycles per second), while the rest of Japan uses 60 hertz.
Wow! That's like dividing the country into left and right hand steering! I just found this very interesting.