The outside condenser should be sitting on a level surface, free of foliage that may block air flowing into it. Check the finned heat exchanger that surrounds the condenser to make sure the fins are aligned and clean. What you want is equipment that is in good shape with unobstructed airflow. Window air conditioners are normally less efficient than central air conditioners. However, because they only cool one or two rooms, their overall contribution to the utility bill may be less than that from a central unit.
Age is especially important in window air conditioners.
Older ones are less efficient and will undoubtedly be more expensive to operate. Check the filters and fins on these units, also. Water Heaters Water heating is normally the second highest utility expenditure. They use energy both in heating water and in maintaining the temperature once the water's heated. The age, insulation, temperature setting, and location of the unit influence a water heater's efficiency.