The huge disaster in Japan has ruined parts of the nation's electrical system, notably the six Fukushima Daiichi reactors that remain shut down. As a result, the country's utilities can't generate enough power to meet demand, so they are using rolling blackouts to give some power to everyone for some portion of each day. That tactic is crippling industry—it's hard to run a factory that makes cars or TVs when the power suddenly cuts out for a few hours each day. The blackouts complicate commuting, so workers can't get to their shifts on time, which further cripples manufacturing. And so on.
Incredibly, the southwestern half of Japan, which largely survived the earthquake and tsunami unscathed, cannot help the northeastern half of the nation, which took the brunt of the damage, because the two sections of the country operate on two separate power grids that are incompatible. As NPR reported on March 24, the southwestern section can actually produce surplus power, but the transmission and distribution system there operates at 60 Hertz, and the northeastern region's grid operates at 50 Hz. This awkward situation, seen clearly on the Japanese map above (blue is 60 Hz, red is 50 Hz), is the legacy of a historic oddity: the "east," as it's referred to in Japan, built its grid based on the German 50 Hz system, and the "west" followed the American 60 Hz system. (An English adaptation of a similar map is here.)
Converting power from one system to the other is a complex task that requires enormous yet highly sensitive machinery. The country has only a few, meager "interconnect" facilities that can do the job, which have nowhere near the capacity to minimize the need for rolling blackouts.
The U.S. has a less dramatic but similarly tenuous setup. The nation is divided into three grids. All three operate at 60 Hz, but again, only a few interconnects exist between the regions. Those sites would have to be beefed up significantly if the country was to benefit from building massive wind farms in the windy high plains or big solar farms in the sunny southwest. More and larger interconnects would also allow regions to "wheel" large quantifies of power between them, to help minimize blackouts caused by storms. Better interconnects would also help utilities that might be in danger of exceeding their capacity (say, Texas, buckling under heavy air conditioning load in August) to get some extra power from another region that has some to spare (perhaps cool Minnesota on that same day).
Map courtesy of Tosaka, via WikiMedia Commons