We used 3.8 TW-hrs in 2005 (which translates to average 434 GW usage, so 500GW is probably a bit low for peak).

DOE (http://www.eia.doe.gov/oiaf/aeo/pdf/overview.pdf) projects usage of 5.3 TW-hours in 2035, which translates to about 605 GW average. We probably need 1+ TW capacity in the US to handle peak usage.