The US also has huge areas with abundant solar energy. They are already building PV installations on the roofs of supermarkets, warehouses and malls. Walmart is self sufficient in energy, in some areas.
Capacitors do lose there charge over time (as do batteries), but this is dependant on construction and type. I have worked with huge capacitors that kept their charge as long as batteries. And as for the amount of energy they can store, that can be enormous. Again it just depends on size how they are configured. Some hypermilers are already using super capacitors instead of batteries.
And we are talking about entire new technology with Graphene super capacitors, so who knows how good they may end up.