The power rationing is necessary due to the country's shortage of electricity.
The electricity grid serving China's financial hub does not have the capacity to meet peak demand the authorities say.
China has been coping with power shortages since March, because of coal supply problems and a drought. When the mercury in the thermometer hits 37C (98.5F) - not that unusual in summer here in Shanghai - power rationing will get under way. Some 24,000 businesses - mainly factories and other industrial plants - will face mandatory power cuts. More >>>