For both an existing and new construction home, the top three items that provide the greatest value and return on investment are as follows:

1. Installation of LED light bulbs and fixtures in replacment of incadenscent or compact flourescent bulbs/fixtures.
2. Sealing of exterior shell of house and all top plate penetrations in conjunction with blower door test.
3. Properly installed insulation in walls and ceilings and upgrading R-Value above code requirements.

Additional items to consider include:

1. Testing and sealing of hvac system duct work
2. Upgrade hvac system condenser SEER and furnace efficiency
3. Tankless water heater
4. Solor photovoltaic system

Currrently, you can receive a federal tax credit for 30% of system and installation costs.

In California, the state rebate incentive has been exhausted and is no longer available.

Typically, the price of your system should depend on how many panels you are having installed.  It is recommended to get a few competitive quotes and always check the reputation of the installers.

Learn more at http://solareenergy.com/pricing/.  This includes some sample pricing information and can help you get a better idea.  Average system size is about 5.5kW and should cost somewhere in the ballpark of $20,000.  This would give you to a $6,000 federal tax credit.

Solare Energy has been serving San Diego for 25 years and provides our customers with a free energy audit and rate analysis before designing a solar panel system for their homes.  This allows us to accurately size the system to your energy consumption needs and get you the best value for your investment.

Learn more at www.SolareEnergy.com or call 858-547-4300 for San Diego area solar installations.

Rob Shaw
Marketing Manager
Solare Energy, Inc.
San Diego, CA

Great insulation plays a key factor in energy savings. There are many different options available to increase the energy efficiency of your home.  James Hardie Siding is specifically designed for the climate in which it is installed. Whether you live in a climate with freezing temperatures, snow & ice or a climate with high humidity, heat, and rain; James Hardie has a siding product built for your homes specific needs. Technology has changed tremendously over the years so homes built 15- 20 years ago may have been installed with what was considered cutting edge at the time. Window replacement and the new technology that windows have now can save a great deal of energy costs. A good product that we can recommend also would be the Mezzo Vinyl Window or Integrity by Marvin Windows. These are high performance windows that are engineered to provide outstanding energy efficiency. Major appliances such as heating and A/C units can also offer great energy savings. 

I say insulation should be at the top of your list. I would also take a look at your water pumps.

Here's a few of the items we actuallyp ersonally suggest when going green: http://xtremegreenhomes.com/green_options.php.

Oftentimes the attic is the place to start.  Make sure you have plenty of ventilation, both intake and exhaust.  Check the soffit cavity to make sure you it isn't clogged with insulation.  There are formulas for Square foot of attic in relation to ventilation.  If you have a vapor barrier in the ceilling a ration of 1:300 sf is ok, otherwise about 1:150 is the rule. Second, check the insulation.  All air escaping into the attic should be sealed using spray foam or other sealants. Next, depending on your geographic location, make sure you have plenty of insulation in the attic.  Finally, something as simple as installing blinds over windows can be an economical solution to saving on energy costs.  As recommended in some of the other answers, an energy audit or getting advice from any number of qualified professionals that are willing to visit your home is the best place to start.