If you have a generator you should only need a few minutes worth of UPS time. Especially if it is auto starting.
Another UPS to consider for long runtimes is a Powerware Ferrups. I have a Powerware Ferrups Tower UPS
at work. It is running at 6kVA load right now and it lasts around 50 minutes. When it was new it lasted well over an hr, it is about 5-6 years old now. We have frequent blackouts and no generator so the long uptime is very handy. I cant remember what kVA it is rated but I know they come up to 18kVA for that model. If you are fealing very crazy they do make models that are 750kVA+.