There are guidelines available to help homeowners estimate the savings in heating and cooling costs that can be realized by replacing old inefficient windows with new high performance energy efficient windows. Homeowners may obtain a reasonable estimate of savings from replacing windows by using one of these guidelines. However, it is important to understand that these guidelines have limitations. Guidelines are designed to provide a reasonable estimate. By their nature, they are simple to understand and quick to calculate. These guidelines are not intended to be an exact measure. However, they are useful to get a reasonable estimate of potential savings in energy costs. How close each individual homeowner’s actual savings is to the amount estimated depends on how close their specific circumstances are to the assumptions inherent in the guideline. Four of the major areas where a homeowner’s individual experience may vary from the assumptions inherent in guidelines are examined in the following discussion.

Replacement Window Product Specifications

Technical performance standards for replacement windows are independently established and certified. Energy Star® Qualified is the most common and recognized standard of energy efficiency. There are minimum standards that must be met for the Energy Star® Qualified rating. However, some manufacturers’ products exceed these minimum standards. Actual performance standards are brand specific. Products must be tested and certified by an independent laboratory. When the performance standards of the replacement windows exceed the performance standards inherent in the guideline, actual savings will exceed estimated savings. When the performance standards of the replacement windows are less than that the performance standards inherent in the guideline, actual savings will be less than the estimated savings.

Energy Inefficiency of Windows Being Replaced

Each guideline includes some inherent assumption about the level of inefficiency of the old windows. Some windows will be more energy efficient than the average used by the guideline. Some windows will be less energy efficient than average. Windows that are less efficient than the assumed average will realize greater savings than estimated by replacing the homes windows. Windows that are more efficient than the average will realize lower savings than estimated.


As a percentage of total energy costs, homes located in climates that experience extreme temperatures experience higher than average heating and cooling costs. Homes in locations with extreme weather conditions will likely achieve greater energy savings than estimated using the industry guidelines when old windows are replaced with new energy efficient windows. Homes located in mild climates will likely achieve lower energy savings that estimated by industry guidelines. Reducing heating and cooling costs and increasing energy efficiency are important factors when evaluating the advantages of replacing windows. Industry accepted guidelines to estimate potential savings are helpful tools when used to calculate a broad estimate of savings from replacing windows.