Thing is, although the total number for tha nation would exceed demand, each parking lot probably wouldn't have enough panels to power that many homes. You could have 5 parking lots within 10 miles but they may only be big enough for you to get 200 homes of power out of them individually, if not less. OK powering small parts can be good because then you have less energy demand for you larger scale power source such as nuclear. Look at London - It has a densely populated central area with lots of big buildings and cars & people. Greater London on the other hand has a lot of people but they are more evenly spread about and have generally smaller houses and less power demand. Using Solar of Wind Power to give electricity to those homes means that you'd need fewer nuclear power stations to give Central London its energy. This is cheaper and more efficient. Of course, Solar Panels do have to be renewed in time, but so will Nuclear Power Stations once Fusion Reactors are efficiently created.