US President Donald Trump said imposing solar tariffs would help US companies. But China’s surprise decision to slash installations is negating those gains, and shares of American manufacturers are plunging. First Solar Inc. has lost about a quarter of its market value in the past week. SunPower Corp. is slumping too, as are the big Chinese solar companies that trade in the US and Asia. China’s cutback “has shaken global markets,” Joseph Osha, an analyst at JMP Securities LLC, said in a research note Wednesday. “Pricing pressure is likely to accelerate outside of the US, and even inside the US pricing may decline as Chinese suppliers look to compensate for a sharp decline in expected demand in their home market.” The industrywide slide shows that trade barriers designed to protect specific companies or industries can be overpowered by the global forces that drive the markets. Slower demand in China will exacerbate a panel glut and drag down prices. That will mirror the conditions that prompted some struggling US panel makers to seek tariffs last year. Chinese solar power stocks plunge as government moves to contain industry size First Solar had been riding high on strong demand and pricing for its panels. In October it announced record orders as US trade policies drove up demand for its panels. On June 1, China pulled the plug on global demand by eliminating about 20 gigawatts of expected domestic installations this year, according to Bloomberg New Energy Finance. That will reduce global prices 35 per cent, to 24 US cents a watt by year-end. The pullback prompted analysts to downgrade First Solar. The company has slumped more than 16 per cent since China’s announcement.