Either you are a savvy consumer and do a through inspection of your home, hire a Energy Audit Company, or receive an audit from a company with the expectations of a sale. You have to look at what is the long and short term return on investment to make any purchase or upgrade worth it.
Some things just do not have the pay off. For instance, the pay off is not there just to go out and buy a new washing machine just because you will save $44 per year unless yours is broke. I have not been able to get solar panels to make sense in my household yet at current cost vs. return.
I have a link to a web site that I have used personally and recommend to help you look realistically at ROI if you are thinking of replacing or upgrading your HVAC system, appliances, windows or making other energy improvements. It can may be helpful in making that decision based on real world figures that make good dollars and sense for your household.
It will ask you a series of questions starting with your zip code, you may start, stop and save.
Link to Lawerence Berkely National Laboratory below.
Pictures courtsey of Google Images.