Buyers looking at the new Ford Focus Electric will have a chance to go even greener by adding an optional solar charging system. Ford is partnering with SunPower Corp., Silicon Valley’s largest supplier of solar power arrays, to provide a 2.5 kilowatt rooftop charging system that could replenish a drained lithium-ion battery pack in about 10 hours. After a deep discount of more than 40% off the normal price tag, the rooftop system will still cost $10,000 after receiving a federal tax credit. Charging a Focus Electric from the grid each day would cost about $10 a week, making the payback period for the SunPower system about 20 years.