SolarCity, the company co-founded by Elon Musk, now produces the world's most efficient rooftop solar panels with a module efficiency of just over 22 percent. That bests the X-Series panels built by SunPower, which top out at around 21.5 percent. The results were confirmed by the Renewable Energy Test Center, a third party California photovoltaic testing company. SolarCity CEO Lyndon Rive told Fortune that the company is shooting for a cost of around 55 cents per watt. It plans to build the 360 watt panels its 1GW solar panel plant in Buffalo, New York, set to open next year.