Having sweltered through May, southern and eastern China face more weeks of unrelenting heatwaves, putting power grids under strain as demand for air-conditioning soars in mega-cities like Shanghai.

In the next three days, most of southern China is expected to be hit by temperatures of more than 35 Celsius, with temperatures in some areas exceeding 40C, according to national forecasters on Friday.

Like many parts of Asia, China has been besieged by extreme hot weather in recent weeks ahead of summer proper in the northern hemisphere.

On Monday, Shanghai endured its hottest day in May in more than 100 years. Provinces in the south sweltered in blistering heat, testing power grids as demand for air-conditioning spiked among home, commercial and industrial users.

Temperatures at 446 weather stations across the nation reached or exceeded the highest ever for the month of May, deputy director of the National Climate Centre Gao Rong told a press briefing on Friday.

Nationwide temperatures this month will be about the same as a year ago, but in parts of the Yangtze River delta, including Shanghai, as well as parts of southwest Sichuan and Yunnan, temperatures will be 1-2 degrees higher, Gao said.

"I'm not surprised that they are occurring, and not surprised that they are worse. But how they are occurring - it's just been week on week on week of these records being shattered," said Sarah Perkins-Kirkpatrick, a climate scientist with the University of New South Wales. "It's just relentless."

Demand for electricity in southern manufacturing hubs, including Guangdong, has surged in recent days, with China Southern Power Grid, one of the country's two grid operators, seeing peak power load exceeding 200 million kilowatts - weeks earlier than normal and close to historical highs.

In recent days, the power load in Hainan rose above 7 million kW for the first time, and also hit record highs in Guangxi, according to state media reports, with further rises expected in other southern provinces including Yunnan and Guizhou in the coming days.

In the summer of 2022, extreme heat in China pushed authorities to ration power use. Hydropower output, key in giant provinces such as Sichuan, was also hit due to prolonged drought conditions.

Powerful convection weather has also wreaked havoc in central China in recent weeks, with protracted downpours and even hail devastating the country's ongoing wheat harvest. In Henan province, known as the granary of China, moderate to heavy rains are expected to continue till at least June 4. (Reuters)