Less than two weeks ahead of an election for the upper house of parliament, surging electrical prices are making life tougher for Japanese customers, following higher fuel costs brought by Russia's invasion of Ukraine.
A power shortage could batter the fortunes of the ruling Liberal Democratic Party (LDP), already under fire for its handling of higher consumer prices.
"Electricity demand has been staying above yesterday's forecast since this morning, amid unseasonably intense heat," an official of the Ministry of Economy, Trade and Industry told a news conference, adding that demand had outstripped predictions.
By 1 pm, temperatures in downtown Tokyo stood at 34.7 C (94.46 F), while in Sano, slightly to the north, they were at 38.6 C (101.48°F).
Ministry officials had warned reserve generating capacity would drop as low as 3.7%, close to a minimum of 3% that ensures stable supply, from 4:30 pm to 5 pm (0730 GMT to 0800 GMT) on Monday in Tokyo and eight surrounding prefectures.
"The supply-demand outlook, based on the latest weather forecast and other factors, is still expected to remain below the 5% reserve ratio during some evening hours," the official added.
If temperatures rose further and demand increased, or power supply was suddenly disrupted, the ratio could fall below the 3% minimum, he said.
"We encourage people to cut back on energy use to a reasonable degree," deputy chief cabinet secretary Yoshihiko Isozaki told a news conference.
He advised turning off lights not in use, limiting use of air conditioners and guarding against the risk of heat stroke.
Weather officials said the end of the annual rainy season across much of Japan, including the Tokyo metropolitan area, had come at its earliest since the data began to be kept in 1951.
Summer heat starts with the end of Japan's rainy season. In 2018, the June 29 end of the season brought an extremely hot summer with several heat waves that put thousands in hospital.
Though expected to do well in the election, Prime Minister Fumio Kishida's ruling LDP faces headwinds from rising prices, worsened by a weak yen currency.
Support for his government, while still strong, has been edging lower in opinion polls, with a majority of voters unhappy about its efforts to rein in prices.