The southern façade, made of about 100,000 square feet of photovoltaic panels that convert sunlight into electricity, collects enough energy to provide up to 40 percent of the building's needs. Costing at least 20 cents per kilowatt-hour—several times as much as coal or natural gas—solar PV is expensive today. But the trends are good: Solar is getting cheaper, and the relative economics will improve as more states and countries regulate the production of greenhouse gases.