Indeed. I just looked up the heat loss for my house: 213W/ deg K. So, to maintain 20 deg C inside with -20 deg C outside requires about 8.5kW of heat energy (200m2 single-family house built around 2008), or approx 200kWh/day.
Generally by using the dimensions and heat conduction properties of the exterior surfaces of the house (area of foundations, walls, windows, roof), taking into account the heat gain due to sun, with some local fudge factors applied (loss due to wind, natural ventilation, outside temperature).
Then, after the building is actually built and inhabitated, the calculations are adjusted by the actual energy consumption over year).
As pointed out, competent HVAC companies should have people on staff comfortable with such calculations. However, my experience shows that it is not universally true, and many are just guided by intuition/experience with other projects (i.e. the roof insulation thickness on the previous project was X, so that's good enough for you, or "well, on average we recommend 50W/m2 of heating power when selecting a heat source"). Which probably works fine for many cases (e.g. renovating an older building, where even if the material properties when they were new are known, you can only guess the values after 20 years of service).
Even a very well insulated home can use quite a bit of electricity.
The most rigorous standard for home efficiency, the Passive House standard, stipulates that no more that 15kWh/m^2/yr is used for space heating. For a 200m^2 house, that's 3000kWh/yr.
Given a 120 day (4 month) heating season, that's 25kWh/day average just for space heating. Obviously it varies quite a bit, with some days much higher and others much lower. Add in other electricity uses, like refrigeration, laundry, and you're easily at 40+ kWh/day even in an efficient Passive House.