Real Power Value (KW) and Apparent Power Value (KVA)

KVA.

Both... for they represent different things. What you need to use when designing anything is dependent upon what it is you are designing.

You need to analyze the nature of the system that you are working on and factor in all parameters that will impact its design.

For example... a large bank of purely resistive heaters used in a melting pot would have no KVA component and therefor its design would be based solely on the load in watts.

Where as a large machine with many inductive loads would have a large KVA component that would have to factor into its design.

When designing an entire system, all of the various loads must be factored together to come to an understanding of the required nature of the whole system.

In this business, as is often the case, there is no one answer for all situations.