Estimating power demand is combination of science and art. It is an area of electrical engineering where there is no correct answer. Plug the figures in your preferred method of calculation and then as an engineer you need to relay on instincts to say if the answer feels right or not.   Depending where you are, different methods, figures and procedures are used to estimate the power demand of an installation. This is a look at one method inline with what could be considered IEC practice. To get going it is useful to understand some basic definitions:

diagram reproduced from Schneider's 'Electrical Installation Guide - According to IEC International Standards'

• voltage V - the voltage of the electrical system
• load current I_b - the current required to operate an item of equipment
• apparent power kVA - the product of the voltage V and load current I_b
• real power kW - the actual power consumed by the load or equipment
• power factor - the ratio of the real power to apparent power (kW/kVA)
• utilisation factor k_u - see below
• simultaneity factor k_s - see below

In estimating power demand this is normally carried out using either apparent or real power.  I prefer real power as it gives me the actual kW required.  Many people will use apparent power.  As we are dealing with estimates (ball park figures even),  using real or apparent power will yield usable results.  In this post I will stick with apparent power.   

Utilisation factor k_u  - name plate ratings invariably list higher values of current than will be seen in use,  motors rarely run at full load, etc.  A utilisation factor can be applied to these ratings to establish a more realistic load current, thereby not overestimating the demand.

Simultaneity factor k_s - not all equipment runs a the  same time; for example one motor may be duty and the other standby.  The same applies to installations;  for example a group of houses or apartments will not  all consume the full design current at the same time.  Applying a simultaneity factor takes care of this.  Often the term diversity is used  and has the same meaning.

The diagram illustrates how the utilisation and simultaneity factors are used to estimate the power demand of an installation.  Click on the image for a larger version.

Following the diagram, the apparent power of the load or equipment is multiplied by the utilisation factor to give a realistic power demand to be supplied by a distribution board.  Summing these power demand figures gives the total connected apparent demand (at that board).  The distribution board would normally be sized for this demand.

An appropriate simultaneity factor is applied to the connected apparent demand at the distribution board and this [diversified] load is carried upstream to higher levels boards.   Repeating this procedure will lead to an expected total demand for the full installation.

In a nutshell, that’s all there is to it - in principal at least.   There are often problems in deciding what simultaneity factor to use and here experience can be really useful.  For anyone who is interested in a little more depth, we have a couple of pages  on the site looking at the same topic:

Maximum Demand Estimation - Wiki Page

Maximum Demand for Buildings - Post