I was googling for some estimate of available roof space, space over parking lots, and space over highways suitable for solar power.  This study of San Diego county concludes that 53% of electric power could come from solar cells if all available, suitable roof space were used.

Double the assumed efficiency of around 10% for older generation solar cells and supply meets demand.  That 20% figure has been reached with several PV designs.  And 38% has been reached with 10 sun concentration in concentrating collectors in National Renewable Energy Lab tests.

By using the latest. 55% efficient wide spectrum cells, going into production now, combined with concentration and the added efficiency of heat collection as well, up to 70% could be possible.

And that solar heat can be used for asdsorption cooling for air conditioning as well.  Air conditioning is the grid breaking brownout load for electric power.  The solar panels also shield roofs from extra sunlight that tends to boost air conditioning load.

Adsorption cooling and direct geothermal ground circulation cooling would reduce air conditioning load on the grid to a tiny perrcentage of current use.

This roof area solar energy estimate is very conservative.  It is likely that with technologies in development right now, sunnier areas of the nation like the southwest could produce far more electricity  than  they use just from solar power mounted on available rooftop and over parking lot space.  That is with no use of space over highways at all.

And highways in very hot regions could generate signifigant power from heat energy harvested with tubing running in the road surface that collects that heat to run electric generating turbines running on refrigerant that recondenses by using geothermal cooling.

The southwest can be a net solar power energy exporting region.  And almost every region in the uS can at least supply it's needs with solar on roofs and over parking lots.  That is even with a huge new power load from electric plugin hybrids.

Concentrating solar cogeneration systems that generate electricity at much higher efficiency than older style flat plate collectors, grow algae that supplies biodiesl fuel, and collect heat energy for heating/cooling; all from the same solar system.  With energy conservation from geothermal heating and cooling, plugin hybrid cars, and energy efficient appliances and buildings  the world can be powered from rooftops, with no extra land space used.