Your ENERGY STAR heating and cooling system must be sized and installed correctly to run efficiently. Many consumers will buy extra-large central air conditioners because they assume the bigger the unit, the better it will operate. However, that is faulty logic and it's actually the opposite that is true.
An over-sized unit will cycle on and off continuously, which increases your electric bill and burns out the condenser. You do not want a heating and cooling unit that is too small either. Smaller units cost less, but will not be able to heat and cool sufficiently. The best advice is to call a service contractor who knows ENERGY STAR products. The contractor will calculate the cooling and heating needs of your home and help you choose the right system.