The equation works only for uniform sections of the home. For example, you can estimate years to payback for a wall or several walls that have the same R-values, if you add the same amount of insulation everywhere. Ceilings, walls, or sections of walls with different R-values must be figured separately. Subtract the areas of windows and doors when estimating paybacks for wall insulation. The cost of the energy source is also a key factor in determining payback.
Energy prices vary widely from region to region, and from season to season.
Other factors, such as the rate of production and inventories of fuels nationwide, can also affect local energy prices. Also, the weather from year to year varies, so your energy costs from year to year will vary as well. Typical AFUE values are 0.6 to 0.88 for oil or propane furnaces, and 0.7 to 0.95 for natural gas furnaces. Older systems are usually less efficient. This information can usually be obtained from your local weather station, utility, or oil dealer.