What power should a electric boiler be when heating an apartment or house?

It is important to properly dimension the power of the boiler and heating bodies – according to the heat loss of the building, but our recommendation is that the power of the boiler always be greater than the power of the installed heating bodies (≈20%). There is no need to be afraid of increased consumption – which is explained in the answer to the question "Does a boiler of higher rated power consume more electricity ?".
This’ power reserve " is good to have because of possible poor voltage conditions (lower than prescribed) at which the actual boiler power is less than nominal. When the power of the boiler is equal to the power of the installed heaters, and the voltage is lower than prescribed, it may happen that the boiler is not able to heat the space even though it is completely correct.
A calculator is set up on the site to calculate the required power of the boiler to heat your facility: Boiler power calculator.

The required power of the boiler depends on multiple factors:
Is the boiler necessary for heating the apartment or house ?
II-what is the quality of the isolation of the object ?
3 – What is the height of the ceiling of the object ?
Q-What is the desired temperature ?

Consider the power of the boiler obtained through the calculator as orientable, and leave the exact calculation to a professional person: the designer or the installer of heating installations.