The earthquake on 11 March triggered the automatic shutdown of reactors at 11 of Japan's 54 nuclear reactors, with a total capacity of 9.7GW.
At the moment, only 18 nuclear reactors are producing power. The rest have either been shut down because of safety concerns or for routine maintenance.
In addition to the nuclear reactors, thermal power plants with a total capacity of 9.4 GW were also shut down following the natural disasters.
In total, Japan's power supply capacity in the affected area has been reduced by about 40%, which is almost equivalent to the national capacity of Switzerland or Austria.
In terms of its energy supply, Japan is isolated, having no interconnections with neighboring countries. The national transmission system is divided into two separate frequency areas - the 60 hertz (Hz) western system and the 50 Hz eastern system. Although the two areas are interconnected using three frequency converters, the total shared capacity is comparatively small (roughly 1 GW). Thus the east of Japan, which includes Tokyo and the tsunami-hit areas, has faced serious power shortages.
In the first weeks after the disaster, the government and electricity companies asked all electricity consumers to voluntarily reduce their energy usage. In addition, rolling blackouts were implemented to try to balance supply and demand.
Electricity companies have taken many measures to restore power supply, including repairing damaged thermal capacity (fossil fuel power plants damaged by the earthquake and tsunami), bringing back thermal plants which were closed for inspections, and also thermal power plants that were previously decommissioned. Thanks to these actions, the supply-demand balance has improved and large-scale blackouts have been avoided.
However, the real challenge is in the summer months - the peak period of power demand in Japan, when temperatures in Tokyo routinely exceed 30C and air conditioning accounts for about 50% of total electricity consumption during peak hours.