We've tried small generators as backup to small
UPs units, and they
don't get on. Big ones are OK, that's the sort of thing they're
designed for.
What do you mean by 'large' and 'small'? For a VCF-like event, I would
expect one of those large units that's trasported on a lorry, the sort of
thing you use for a large outdoor concert, or an entire (large) building
site, not one of those little things you pick up with one hand and give
you 13A (@240V) if you're lucky.
-tony