Re: Recomended generator
Is the phone system on the UPS ? If not, I agree that you have to have that load figured also. Unless you have some high powered phone switch, I can't see that being more than a couple of kW if even that. A 60 kW gas genset will more than drive the 18 kVA UPS and both will be happy. Today's generators and UPS's work and play well better than they used to thanks to improvements in technology. Used to be the general rule of thumb was triple the UPS size to get the genset size when using nat. gas or LP and double it for diesel. I'd bring your UPS supplier and genset supplier all to the table together and hammer out the sizing so that all are happy and then get it in writing.
Good luck.