Maximum Demand

Google will do this for you

If you have a 1 line diagram showing all of the devices downline from the primary connection, you can just survey what the sum total of all the loads adds up to. Of course, you could look at the current ratings of all the circuit breakers in each of the panels on your 1 line diagram. That should be a rather conservative figure because the probability of every panel being fully loaded at the same time is fairly low. Not impossible, but unlikely.

Bare in mind that some panels have more circuit breakers than the main will allow (at least theoretically).

In a nut shell, you have to look at the entire power distribution circuit.

If your question had been in a marketing forum, I would have referred you to a crystal ball!

Add up all the loads that might be running at the same time, plus starting current for the largest motor.

In my region, the meter records the consumed (kVA) continuosly. There is a sanctioned max demand for HT customers. If that is exceeded and remains so over a period of 30 minutes, it is recorded by the meter and the customer is charged accordingly.

Contact your utility and do it the same way they do. If you do it any other way your results will vary and will not be the same as the utility's. The posters above answered the question "How do you estimate maximum demand?" which is usually an entirely different matter.

For planning a new installation....Go to the relevant Standards \ Codes for your area, or get an Engineer to work it out for you...

For an existing installation, get a report from your supplier or have Data Loggers installed for a month to record the site usage, this will eliminate all the guess work.

Regards,
Sapper