Keeping your refrigerator full, but not to the point of overstuffing, will allow it to run at max capacity. To help retain cold temperatures when the fridge and freezer are less full, place glass or ceramic pitchers of water in their compartments. Like using ice in a picnic cooler, this will insulate the interior and keep it cold.
Refrigerators and freezers account for about a sixth of all electricity use in a typical American home, and they use more electricity than any other single household appliance.
Over- or under-crowding makes the fridge work harder to maintain its optimal internal temp (37 degrees F).