Move forward into the 1980's
Work in kW.
The calculator will probably tell you the heat lost when maintaining the indoor temp 20C above the outdoor.
But to warm it from cold takes more.
To warm it on a cold night when outdoors is -10C takes more
To warm it quickly takes more.
Radiators keep the rooms more comfortable if they are long and warm than if they are short and hot.
Builders use the calculators so they can save money and increase profits by using the smallest and cheapest qualifying heating possible.
Modern boilers usually have more than enough power, so instead of running noisily at max, they modulate down most of the time and tick along quietly. You may be surprised that a 24kW or 30kW boiler can cost the same as a 12 or 15, or less. It is often the same boiler.
I'm in favour of having radiators 50% bigger than the calculators say, for improved comfort and the ability to cope with unusual cold spells.
Thermostats and TRVs prevent your house from being overheated and using more energy.