A big long-term question hangs over Hong Kong's proposal to import power from the mainland: climate change.
Projections indicate southwestern provinces that make a major contribution of hydropower to the China Southern Power Grid could lose up to 10 per cent of their water within three decades because of droughts caused by global warming.
And that, says a mainland environment scholar who has studied the likely effects, would have a big impact on the ability of the provinces, principally Sichuan and Guizhou , to generate electricity.
"If the available water resources decrease by 10 per cent, it would considerably affect hydropower potential and hydropower generation," Professor Tang Qiuhong said in an e-mail reply to questions from the South China Morning Post.
Tang, an academic with the Institute of Geographic Sciences and Natural Resources Research at the Chinese Academy of Sciences, said Yunnan province would also be affected by global warming, although to a lesser extent.
His comment comes as Hong Kong ponders the option of importing 30 per cent of its electricity demand - estimated at 15 billion kilowatt-hours by 2023 - from the southern grid versus an option of boosting its own natural gas-fired electricity generation from the present 22 per cent to 60 per cent.
Tang said his colleagues at the academy were now studying how serious the droughts in the affected provinces would be.
The study also found that within the period, 40 per cent more people globally would be exposed to the risks of "absolute water scarcity" - a term meaning that the amount of renewable water available was less than 500 cubic metres a person.
In 2012, the southern grid received about 37 per cent of its power from hydroplants in the southwest, but this is already proving unreliable in times of drought.
That year, a record 124.3 billion kilowatt-hours was sent from these provinces mainly to Guangdong, where a heavy manufacturing base spurred high demand.
But in the previous year, the amount transmitted was down more than a fifth because of a drought that caused power shortages across the south.
In that year, Guangdong also imported 13 per cent more electricity from CLP Power in Hong Kong, compared with 2010.
Long-term plans for the grid call for generation capacity to grow 60 per cent from 7,530 megawatts in 2012 to 11,940 by 2020.
Hydropower will continue to play a vital role, with the share staying at about 37 per cent to 2020 while fossil fuel's share drops from 56 per cent to 47 per cent and nuclear triples from 3 to 9 per cent.
Hong Kong now buys about 10 billion kilowatts a year from the Daya Bay nuclear power station in Shenzhen.
Environmentalists say they support the import option - but with reservations.
Veteran Greenpeace campaigner Prentice Koo Wai-muk said the green group backed imports although it was aware of the rising nuclear share.
But he said signing a contract with the southern grid simply amounted to bringing in one more company to dominate the market. "The import agreement should be one that opens up opportunities for Hong Kong to source clean and renewable energy," he said.
Koo said the mainland had started experimenting with allowing big users to get power directly from the producers and the grid in future would serve only as a middleman.
Dr Billy Mak Sui-choi, of Baptist University's finance department, said power imports could give Hong Kong a diversified electricity source, which was vital to maintain high reliability.