What to look for when purchasing a power generator?

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http://serverfault.com – After an extended power failure that caused our UPS to run down and have issues coming back up once power was restored, I'm looking into getting a generator. What do I need to look at when specing a generator? I know I will have to calculate how much power is actually being used. I'm not sure how to factor in the UPS units deciding to start charging their batteries back up as soon as the generator kicks in. I'm not certain how to tell what generator+UPS combinations will work properly either. I also have to choose between diesel and natural gas - what are the pros and cons there? I'm in (HowTos)