Japan could have a power shortage of 9.2 per cent next summer if all of its nuclear reactors are shut.
All 54 of Japan's reactors could go offline by May 2012 if safety fears over nuclear power following the Japan earthquake and Fukushima Daiichi nuclear crisis delay their restart after regular maintenance.
If Japan had to operate without any nuclear reactors it would only be able to supply 162,970 megawatts of power next summer, which is 9.2 per cent short of estimated 179,540 megawatt peak demand, according to government estimates presented to a ruling party meeting.
"We'll have to overcome the power shortage with more power savings if we have no hopes for nuclear plants' restart," said Hirofumi Kawachi, senior analyst at Mizuho Investors Securities.
"Some people say that we can cover the 10 per cent with power savings, but the fossil-fuel plants have been operating at full capacity this summer, so we do not know if we can do the same next year."
A government official said a team was working out the estimates and that the final forecast could be different as they were taking into account constantly changing factors.
Separate research by the Institute of Energy Economics for Japan (IEEJ) estimated that Japan's power supply without any nuclear reactors would fall 7.8 per cent short of an estimated 182,000 MW peak demand next summer.
The energy forecaster is predicting the shortage despite its assumption that utilities will boost run rates for existing coal-, oil- and gas-fired plants to 85 per cent, almost 100 per cent and 70 per cent, respectively.
Both simulations do not take into account the effect of power savings, which so far have spared Japan electricity shortages this summer, when demand peaks because of the widespread use of air conditioning.
Only 16 nuclear reactors are now operating in the wake of the natural disasters and the share of nuclear power in the nation's energy supply fell to about 18 per cent in June from about 30 per cent before the Japan earthquake and tsunami.
However, a combination of mandatory power cuts and voluntary savings by companies and consumers allowed the utilities to match supply and demand without resorting to rolling blackouts.
Some industries have opted to operate on the weekends to avoid peak hours and sales of electric fans have been brisk this summer as many households have reduced usage of air conditioners to save electricity.
As part of a broader campaign to stave off power shortages the government is considering a plan to equip households with smart power meters which track consumption for more efficient use of electricity..
Tokyo aims to get those meters to track about 80 per cent of total energy demand within five years.