Written In general, a 5 Ton A/C unit will require 18 kw for running and perhaps 54 kw to start up. Your generator should be 17 x 18 + 1 x 54 = 360 kw provided it is fairly close to the A/C  units and there will not be any serious voltage drop in the wiring connecting the units, and the units can be controlled so that only one will start up at any one time. Note also that the indoor air handling units are not included.  See specification sheet here. Classic Series: Package A/C (3-5 Ton)Updated Written I suggest you get somebody to size your generator. The information provided is very basic for sizing a generator. It's important to understand the following for you to get a properly sized generator:Start up current of your AC units?Nominal current of your AC unitsStart up sequence of your AC units (do they all start at the same time or are they set up to start up in a specific order)?If they all start at the same time, are you willing to change to delayed start up of some of the 18 units?

This will allow you to get a smaller generator as your start up current will be lower.Correctly Sizing a Capacitor I don't know how many times a technician has said that they installed a part based on what was on their service truck.  I have heard of technicians wasting money over-sizing contactors, cutting down air filters, and even using controlled substances to clear condensate drains!  average cost to replace air conditioning unitOf course, these scenarios all get the job done, but I would argue the many reasons why not to do these.  carrier hvac parts cross referenceThe one thing that gets to me is when a technician doesn't verify they are installing the correct size dual run capacitor.  window ac unit angleBelieve it or not, there is a simple method to figuring the correct size capacitor, without waiting on hold for the distributor's guru. 

Of course, you could use a multimeter that reads microfarads (uf), but this will only tell you if the existing capacitor is weak - not the correct size! Testing Volts / Amps With the condensing unit operating under a load, you will need to measure the total volts between the HERM and COMMON terminals on the run capacitor (i.e. 345 VAC).  Next, measure the amperage on the wire leading from HERM to START on the Compressor (i.e. 4 amps). Use the equation below to verify the size of the capacitor. The resulting microfarad (uf) should match the size of the installed capacitor. An over or under-sized capacitor will cause an imbalance in the magnetic field of the motor.  This hesitation when operating will cause noisy operation, an increase in power consumption, a drop in motor performance and eventually overheating or overloading motors like compressors.  The run capacitor should have the exact microfarad (uf) that the motor is rated for.  Capacitors rated above 70uf are considered Start Capacitors and are generally removed from the circuit electrically during operation.