We can compare building heat loss to that of a tank.
If we don’t add water, the above tank will empty completely. This situation is very similar to that of the house, below, which is losing its heat.
What will be the interior temperature of this house if we do not heat it?
If we don’t heat this house, it will have the same temperature as outside (0 [°C]) (32 °F) once it has lost all its heat.
We call these losses of heat « heat loss ».
Heat loss is calculated in Watt [W] or Kilowatt [kW].