[Big Dave]

MechEngVT has it. It is the intermittent nature of wind and solar that doom them. To back them up you have to run a fossil fuel plant in wasteful "spinning reserve" to cover for when the wind or solar units drop the ball.

We should be working on utility sized energy storage to level out the output of wind/solar. Pumped storage is nice but is highly site dependent and most of the sites have been developed already.

The big Vanadium batteries offer promise but aren't ready for prime time.

Until somebody comes up with a mobile-usage battery, running you car on wind or solar is just out of the question.