The price of solar panels has fallen far and fast. But the Energy Department (DOE) wants to bring those costs down even further, especially for residential homes. After all, studies have shown that if every inch of useable rooftop in the US had solar panels on it, the panels could provide about 40% of the nation’s power demand. Right now, the DOE’s goal is residential solar that costs 5¢ per kilowatt hour by 2030.