Grocery stores without power were very common in South Florida in the years before Francis, Jeanne, and Wilma. Some of that has changed voluntarily and with new building codes. The Publix (popular grocery chain in Florida) closest to my house has one of the huge megawatt generators you described following being out of power for a week in 2005. Our emergency preparedness organizations have published maps showing both grocery stores and gas stations with backup power. Might be worth the effort to see if such maps are published for your area.