Total energy demand of a new building

Hello everyone,

I am interested to know if there is a kind of formula or practical way to calculate or at least estimate the total energy demand (heating + hot water) of a house as a layperson.
I am concerned with the energy that the heating system ultimately has to provide,
so a number that is independent of which heating system or heat pump is ultimately used.

Apparently, the values from the [Energieausweis] cannot be used for this purpose.

Take a look at SHK (Sanitär Heizung Klima). There are also some videos on the topic on YT. There are rough calculators for certain house construction years (heating demand). Otherwise, you could estimate which energy saving regulation/building energy law standard applied in that year and from that estimate the heat energy demand. Some calculators on the internet only ask for: construction year, number of people, living space size. Physically, the calculation would probably be: How much energy do I need to supply to heat x L of water to temperature °C? In our calculation, the warm building envelope (room temperatures) was compared to a cold average outside temperature and the heat losses in total resulted in the heating energy demand.

The values from the energy certificate apparently cannot be used for that.

If there is an energy certificate, then the house is either a new building = a calculation is available or an existing building = data from previous usage is available. All own calculations are as accurate as estimating based on the energy certificate. Especially if you do not even know what was installed.