Should ATS Load Testing Be Simultaneous to Determine Diesel Generator Sizing?

I have an existing diesel generator that I would to replace. Its current size is a 400KW 480v 60HZ. I have 7 autmatic transfer switches and 1 manual swtich for diferent buildings. According to report receive monthly load testing is performed in each individual automatic transfer switch, in order for to determine the existing load or the historical maximum load, should  the testing have been performed in all ATS at the same time to get that historical load? Please let me know.
Thanks

Hi Will-That's actually a tricky question.  If you absolutely MUST be able to start all of the loads at the same time (assuming you begin with power to all buildings off) then you'll need a much larger generator that you would if you implement some load shedding. When I say "load shedding" I mean  implement a system where all of your HVAC compressors can't start at exactly the same time. Also, identify non-essential loads and leave them off or delay startup until the initial surge has passed. You might start by measuring and recording the power requirements for each building and then assess what needs to be done. You can rent very nice power quality monitoring equipment that will not only record current surges and voltage trends, but also things like harmonic distortion and high frequency noise. Are there UPS's that will keep some of the loads going until the generator starts?  That can work in your favor.400KW is one BIG generator.  This is not something to be taken lightly.  Please make sure you've got all of the necessary information before proceeding with this project.-Rick .