Efficient by what measure?
This question is at the heart of the topic at hand. What exactly is, in our opinion the most efficient way of executing a vehicle for it's purpose? Like I mentioned, the Prius uses a solar panel to lower the internal temperature of the car to approximately that of the outside of the car. This can be achieved by rolling the windows down a tiny amount to allow the car to reach a thermal equilibrium with the external temperature. So, does this necessitate a solar powered sun roof? No, in fact you don't even need power windows. Does this nullify the use of solar powered panels on vehicles? No. Does this sell cars because it gives the buyer the idea that it is innovative and efficient when really it's a bunch of bells and whistles designed to blow smoke up their dumb rich ass? Maybe..