Customers say that they want a UPS that will carry the load for an extended period without a generator. But what they really want is a magic box that will make electricity for 12 hours without any maintenance, and without any concern for back-to-back outages.
It was explained to me many years ago that it is usually a mistake to configure a UPS to handle an extended outage without a generator. You wind up with a big expensive bank of batteries that will need to wait for utility power to be restored before they can be recharged. You simply can't handle another outage right after recovery of utility power because the batteries haven't been able to recharge. The big, noisy generator is an important part of the system if the equipment has to keep running.
