Beijing has experienced its first "brown out" of the year - a partial power cut to save energy as China grapples with soaring electricity demand.
Fierce temperatures are putting resources under pressure
It came as China's state media announced that the authorities had approved two nuclear power projects in regions particularly hit by shortages.
Thousands of companies in Shanghai have switched to working at night so there is no need to use air conditioning.
State press articles have urged people to stop wearing suits in the summer.
According to China's Xinhua news agency, Beijing's grid switched off power to parts of more than 10 districts and counties of the capital, after three electricity generators in Beijing, Tianjin and Hebei broke down on Wednesday.
China hopes that the introduction of two more nuclear power plants will help to meet the country's demand for energy, which has soared as its economy has boomed.
The State Council had given the go-ahead for work to begin at the Lin'ao power plant project in Guangdong province and at the Sanmen project in Zhejiang province, Xinhua said.
"Nuclear power generating is a kind of clean and safe way of power supply, with mature technology and good flexibility," the cabinet was quoted as saying.
China has four nuclear power plants, accounting for less than 2% of the country's total energy supply. It hopes to expand that to make up 4% of the total by 2020.
Energy industry officials warned on Thursday that China is expected to see a 250-million-tonne annual crude oil shortage by 2020.