A house-sitter will invariably use less energy that you and your family or housemates — she's only one person. Unplug things (alarm clocks, lamps, blow-dryers, treadmills, etc.) and switch off your power strips in the rooms she won't be using, and try to limit the heat or A/C to the areas she'll use. She can adjust the heat/air as she comes and goes so less is used when the house is empty.
And if you have a security system, teach your sitter to use it. There's no substitute for a good motion detector and a super-loud alarm. Safe travels!