How much will I save if I heat to 21 °C instead of 24 °C?

In my search for an answer to this question, I came across the 6% rule. This is a fairly simple rule that empirically (i.e., from experience) determines that every 1 °C decrease in temperature leads to a 6% reduction in heating costs. This rule has been verified by several different measurements and is generally used in the field of savings. However, it is clear that this is a rough estimate, because if, for example, outside was 19 °C, and we lowered the heating temperature from 20 °C to 19 °C, we would save 100% on heating.

From the example above, it is clear that savings depend on the temperature we heat to and the ambient temperature. Certainly, the savings will also depend on the wall material Insulated walls let less heat escape than uninsulated ones. It certainly also depends on the area and thickness of the walls. The wider and thinner the wall, the easier heat escapes from the house.

However, we kept these influences the same in the question, so in the end, we can consider the 6% rule to be consistent with reality. I'll just add that we could proceed in the same way with air conditioning. This time, we would be addressing the same problem, but in reverse, meaning that heat is transferring from the outside to the inside. And here we could also use the 6% rule.