The manufacture of Chinese solar panels exported to Europe produce a carbon footprint twice the size of those made in Europe, a US study has found.
It says this is because China has fewer environmental and efficiency standards and generates more electricity from coal and other non-renewable sources.
The study was conducted by Northwestern University and the Department of Energy's Argonne National Laboratory and published in the Solar Energy Journal.
"It takes a lot of energy to extract and process solar-grade silicon, and in China that energy tends to come from dirtier and less-efficient energy sources than it does in Europe," said Seth Darling, a co-author of the report and Argonne scientist.
The study analysed the "embedded" energy in Chinese- made solar panels, from mining of raw materials, the manufacturing process and shipping the finished products.
"While it might be an economically attractive option to move solar-panel manufacturing from Europe to China, it is actually less sustainable from the lifecycle energy and environmental perspective," said Fengqi You, assistant professor of chemical and biological engineering at Northwestern University and an author of the paper.
A Chinese-produced solar panel made from silicon and installed in sunny southern Europe would need to be used for 20 to 30 per cent longer than a European-made panel to produce enough energy to offset the higher carbon emissions of its production.
Li Yan , Greenpeace's East Asia climate and energy campaign manager, said China's high use of coal for electricity meant its factories were much dirtier than those in most European countries, where renewable and nuclear energy were more common.
"Although the new findings do not change the fact that solar panels are a cleaner energy choice, it should raise the alarm for Chinese solar-panel makers: the life-cycle carbon emissions could become a problem in the long run as more countries put a price on carbon," Li said.
China is the world's largest maker of solar panels, accounting for more than 60 per cent, according to data from the China Photovoltaic Industry Alliance.
The study's authors propose a carbon tariff on Chinese solar panels of between €105 and €129 (HK$1,100 to HK$1,400) a tonne of carbon dioxide to offset the emissions.
"This could be bad news for China's solar-panel makers who export about 95 per cent of their production, especially as they're still battered with the fight over anti-dumping duties by the European Union and the US," Li said.
China did not have specific environmental and energy limits for solar-panel makers until 2010, when the central government moved to curb the blind expansion of the industry.
Last year the Ministry of Industry and Information Technology updated the requirements, setting limits on energy consumption and pollution emissions. By the end of the year, 109 companies accounting for 74 per cent of solar panels met those requirements. A further 74 companies have since met the target.