"If you live in an area where temperatures commonly reach over 80 degrees, I think the upgrade is a good idea and does add to the resale value," said Realtor Dante Drummond with Coldwell Banker. "If the heating ducts are in and it is easy to connect air conditioning, why not?"

For those who live in the San Francisco Bay Area, air conditioning might not be that important, depending on whether the fog rolls into the neighborhood or not. However, when planning a remodel or thinking about the future, it's a good idea to consider both the heating and the air conditioning for efficiency and energy savings.

Realtor Dale Klippel of Prudential California Realty suggests evaluating the heating and air-conditioning system in the early stages of a remodel.

"In the thinking and planning stages, I would suggest that you consider replacing the current system with that of an air-conditioned system. If you plan to sell this home at some point further down the road or are rehabbing the house for immediate resale, put the upgraded system into the house," Klippel said.

"You never know who may wish this service or who may need this service. It might be that an elderly person or couple who, due to their health, might consider an 80- to 85-degree day outside a health problem. If this person or couple have a choice between two homes, they will go with the one with air conditioning all the time. In fact, they most likely will pay a little premium on the home offering air," Klippel added.

For buyers who are considering the purchase of a home that has either space or wall heaters, Klippel recommends consulting a professional heating company to see how much it would cost to replace the old system with a central forced-air system that offers both heat and air conditioning.

"This will improve your value, and even if you only need the air for less than 30 days all year long, it's the comfort level during those days that will help pay you in the long run," Klippel said.

If a homeowner already has an air-conditioning system, it might pay to have a licensed heating and air-conditioning technician check it out. A tune-up of the original system can increase productivity and save on summer energy costs. If the air conditioner is 10 or more years old, a new energy-efficiency model might be the best investment.

According to PG&E, a technician should inspect and test all electric panels, breakers, fuses and wiring. An air-conditioning tune-up consists of cleaning the coils, straightening the coil fins, oiling the blower and checking the airflow and refrigerant level. The technician should follow a written step-by-step procedure and submit a written report on the system before and after servicing. A poorly maintained air conditioner uses 10 to 30 percent more energy than necessary.

PG&E says air conditioners work much like a refrigerator. A fan blows air across a cold evaporator coil and then sends this cool air in the house through the supply ducts. Warm air is drawn into the return ducts and sent back into the system to be cooled. A refrigerant takes the heat to the outside unit, where it is dissipated to the outdoors.

If considering the purchase of a new system, it's important to make sure it is the right size for the home. A unit that is too big for the home will waste energy and money. If replacing an old unit, pay attention to several factors that could impact the selection process, such as whether or not the home's efficiency has been upgraded through added insulation or the installation of high-performance windows. Also, take note of whether trees or other vegetation have been planted to provide shading to the house and windows.

Experts suggest buying a unit with a Seasonal Energy Efficiency Ratio (SEER) of 13 or greater and an Energy Efficiency Ratio (EER) of 11 or greater. The EER is a measurement of how energy efficient a unit remains during a specific time of ongoing use. Use the SEER to compare different air-conditioning units. Air conditioners range from 9.7 to 17, and the higher the SEER, the more electricity can be saved. For example, going from a SEER of 6 to a SEER of 12 could cut cooling bills in half.

PG&E is offering rebates of up to $500 on qualifying central air conditioners and heat pumps. For more information, visit http://www.pge.com/foryourhome.