If you have an electric water heater it will use less electricity when you are away than if you were home using hot water. If an electric water heater is left energized during your vacation it will continue to maintain the tank temperature, even if you’re not using hot water.
Heat is lost through the insulation and copper pipes that come out of the top. If you are going to be away for more than a few days, you might want to consider turning your water heater off at the panel box.