You need to be careful about getting a way-oversized UPS to get long runtime. The light-load efficiency is not that great, so you don't really get all those VA when you are running the UPS light. If you really need to keep your computers up for a long time, you're probably gonna need a generator. There are many small, inexpensive, quiet sub-1000W inverter generators on the market these days. The biggest hassle with portable generators is keeping fresh gasoline available, and not letting the carburetor get gummy/rusty. There are some that run on propane, but last I looked they were all the traditional heavy noisy style.