Which raises a valid point, in that the steady state heat loss rate doesn't automatically define the heating power requirement. There needs to be sufficient excess heating power available to be able to bring the space up to a comfortable temperature within a reasonable period of time. This depends to a significant degree on the heat capacity, and thermal conductivity, of the materials that make up the first 50 to 100mm of the internal structure (ceiling, floor, walls etc). Assuming a plasterboard ceiling, concrete floor and plastered or dot'n'dab boarded walls, then the internal fabric heat capacity could be anything from about 4 to 10 kWh for a 10°C temperature change. If the room started off 10°C cooler than desired, then significantly more heat input will be required initially in order to get the internal fabric up to temperature.