Par of purchasing a home is understanding its energy use. As a practice I research the utility bills of all the homes I list and sell. However the utility bills tells you about the energy and water being used, but to understand how efficient a home is you need an energy audit. Duke Energy provides a free energy audit, or you can contact a company that performs independent audits.
There is also a mortgage product on the market that allows home buyers to roll the cost of purchasing energy saving items into their mortgage. Examples include new furnaces, hot water heaters, and insulation. All of these costs can be financed into the mortgage amount so there is no out of pocket expense and the difference in the mortgage may be offset by the energy savings.
If you want more information on the Energy Improvement Mortgage follow the link to the article.