Sizing of heat pump (large variation on online portal)

Hello everyone,

as already described in another thread, I want to switch to a heat pump at short notice (gas heating acting up for the second time now).

In order to find the right manufacturer/partner at all, I have dealt with the topic of sizing (not so easy, as I have only owned the house for 6 months).

Key points about the house:
- built in 2001
- 150m² living space, of which 40 are in the basement (half window) and currently do not need to be heated strongly, supply line later on
- 65m² usable area
- previous owner's consumption: 15,000 kWh/year (energy efficiency class C)
- underfloor heating in kitchen, living room and bathrooms, thermostats in the basement and upper floor
- currently 2 people, later planned max. 2 children (showering yes, bathing rather rarely)
- 20-21 degrees are usually enough for us, if there are peaks of only 19 degrees on 2-5 days a year that's also okay.

Key points about the area:
Standard outdoor temperature: -10.2 °C
Annual average temperature: 11.0 °C
Altitude: 105 m
Location correction heating load: 88 %

I have now used various online tools, but get totally different sizing results (always heating + hot water):
Federal Association Heat Pump e.V. (by building type): 8.1 kW
Federal Association Heat Pump e.V. (by annual consumption): 12.9 (???) kW
German Energy Efficiency Agency: 10.62 kW

I find especially the last two strange. My house is not huge with 150m² and energy efficiency class C is not bad either. Do you have tips on how I can roughly determine the sizing before purchase (the fine tuning can then be done after a year, just that I choose the right pump now).

Do you have any tips on how I can reasonably determine the sizing before purchase?

1. You ask a professional (e.g. Heckmann) and have everything calculated for you, including the sizing of the heat pump, for a little over €200.
2. You take the previous owner's consumption as a basis (although this consumption can be very individual - a fresh air fanatic with a window always tilted open but still 23 degrees room temperature will skew your consumption).
3. You take data of your house and do your own heating load estimation. Look up "heating load estimation based on the Energy Saving Ordinance calculation / envelope surface method". This estimation is extremely accurate.

not so easy at all, since I have only owned the house for 6 months).

an important question, should the house stay as it is, you don’t want to renovate now and then plan the next measures (window replacement, thermal insulation, etc.) in a year. The heating should surely last for 20 years