After the earthquake, a huge tsunami hit Japan and damaged a few nuclear power reactors meaning that not enough electricity can be generated for the East of Japan given the current power consumption rates. The government split up most areas in East Japan into a few groups and put into effect rolling blackouts. This means that areas will have planned blackouts between certain hours.
Everybody is doing their part to save as much electricity as possible and this can be visually noticed around Tokyo - especially during the evening. Most of the neon signs are turned off and pretty much all the large TV screens are now just blank. One of the reasons however why so many folks are doing their part to save electricity is also because if a shop sticks on their neon lights and TV screens for advertising - they will sure get a bollocking from their customers and general public.
Due to everybody working together, many of the planned blackouts have been evaded as demand is kept lower than the supply but it means that the streets of Tokyo at night are darker than dark.
The reason why electricity cant be channeled from West Japan is written up in the A Week in Tokyo 64 post.