Putting that number in perspective

Recapping...
Saving 10% of energy used in buildings = ~$676B if you tried to replace it with solar PV.

According to the White House Budget Summary for 2010:

• The US Military Budget is ~$707B. That's about 20% of the entire US budget [or about 41% of entire world spending on military].
• Corporate income tax revenues are projected to be ~$179B. Only ~26% of what you'd need.
• The only other two income line items of equivalent magnitude are:
• Personal income taxes (~$1051B)
• Social Security payroll taxes (~$683B) 

Looked at another way: If you spent the ENTIRE GDP of the US (~$14Trillion) at trying to replace the entire energy usage of the US (3297TWh) with solar PV, it would take ~7.9 months of spending (65% of GDP).
To put this number in perspective, the US Military Budget is 4.8% of GDP. That's ~17 days of spending.

Not that it's realistic or even correct, but can you imagine the entire US economic output focused on nothing but solar for almost 2/3 of a year?