Hello Kingsley, you have asked a very important question which is how we can calculate the energy loss in a system. When we can calculate, we can determine the exact amount of energy lost in a given system.
Usually we use an estimate of 20% loss in our system designs and sizing and this is a good estimate as long as the right cabling and efficient machines is used.
However I will recommend you download a separate eBook (PDF) on how you can actually determine the energy loss in a system. You can download it here: https://www.powersystem.org/docs/publications/c1-triplett2010-%5Bcompatibility-mode%5D.pdf