Energy efficiency class!?

Yes, that is important. But for existing buildings, the primary energy demand is usually taken, because it can be easily estimated from the consumption of the past years, which is cheaper for the energy certificates than any other values. On the topic of actual consumption: modern houses are (should be) well below that; with my heat pump I have used about 9 kWh of electricity per year and square meter for heating and hot water in the last 12 months. Without counting the basement, because I only partially heat it.

Thank you for your answers. Unfortunately, I didn't understand anything at all... Can someone answer my question for absolute noobs?

If you do not understand the Wikipedia article... I will not continue talking.
More than spoon-feeding is not possible. Please make an effort yourself.

That is why I use a forum so that it is explained in normal German...

Crude: high value, high heating costs.
A bit less crude: value * square meters = annual energy consumption

A bit less crude: value * square meters = annual energy consumption

And then it depends again on the type of heating, how much the whole thing costs you.