I have not ever been able to make solar installation on my house get a reasonable payback, even with rising electricity rates, the best location in the country for solar, and huge government subsidies. Large solar installations remain a publicity stunt, a sort of really expensive indulgence bought to garner the "green" title:
Scott Gustafson runs the numbers on the solar installation at the revamped Phoenix convention center:
capital cost: $850,000
operating costs: not provided
annual electricity savings: $15,000
return on investment (ignoring operating costs and interest): 1.7%
Solar is still a fine toy for the rich and public figures like Al Gore looking to disguise their true carbon footprint. But the economics aren't there yet for big boy investors -- its still off by an order of magnitude, at least.
Hopefully, this will change as high energy prices encourage innovation.