The calculation acts as a pointer
This calculation is highly simplified and only provides a rough estimate of how much you can save with energy-rated windows. It does not take into account the fact that you can save still more energy by reducing your indoor temperature. For each degree by which you reduce your indoor temperature, you can add savings of approximately 5% of your heating costs to the result of the calculation.
Another and more detailed calculation method is the Life-Cycle Cost Calculation (LCC), which takes into account the energy costs over the house’s lifecycle.