I do not know hw to do the power math, but I was hoping these little guys might draw so little power that solar was realistic:

Koolatron
http://koolatrononline.stores.yahoo.net/koolatron-d07-portable-kool-sport.html

Vector 5 liter
http://www.cooler-store.com/rubbermaid_travel_cooler_and_warmer_149_prd1.htm