British Petroleum's solar power unit is buying the remnants of Fairfield-based Advanced Photovoltaic Systems out of bankruptcy and will invest $20 million in a solar power manufacturing plant to reopen later this year with 100 workers. BP Solar International does not generate solar energy but manufactures and markets panels to businesses, individuals and utilities, BP President Dipesh Shah said. The plant will produce "thin film, glass-based" photovoltaic cells to be marketed nationwide. Currently, photovoltaic cells produce only about 20 megawatts of the state's 45,000-megawatt peak generation load, according to the Independent Energy Producers trade group of Sacramento, because solar power is six times more expensive than natural gas power, the lowest-cost source. But BP expects photovoltaic power costs to drop dramatically in coming years with improved technology driven by heightened global demand for clean energy. BP and other companies are encouraged by polls indicating that significant numbers of consumers would pay more for clean energy if given the option in the coming deregulated energy market. BP's solar power operations produced $100 million in revenue last year, and is the world's third-largest maker of solar panels after Siemens of Germany and a joint venture of Amoco and Enron.