Speaking on Deep Background, the Press Secretary whispered:
Does anyone know of a way to get a UPS to trigger a generator to start, and
to switch over to the generator power automatically or does this type of
thing just not exist?
The UPS will have nothing to do with this. The generator would be
connected to an Automatic Transfer Switch (really an A.T.Relay..)
a) Wait from several seconds up to a minute,
to avoid false starts.
b) Start the generator
c) Transfer the load after the generator is going.
If the UPS is plugged into a generator-fed outlet, it then
goes back to charge.
I *have* seen the issue others mentioned - where the USP would
not recognize generator power. It was SmartUPS's and I'm still
puzzled. The AC output did have some ringing on the scope
but it was not clear that was the cause.
Note a Variac [autotransformer] would be of no help. What might
would be a ferro-resonant ["Sola"] transformer. FR transformers
use a big hunk of transformer iron, in effect a magnetic
flywheel, to smooth out all kinds of dips, spikes and surges. But
there's a heavy price for such - an unloaded 1000 watt Sola draws
~340 watts. Their efficiency is better at rated load, but they
still make nice heaters. They also HUMMMMMMMM.
FR's have another issue. The output voltage is not a function
of input voltage but *IS* a function of frequency! And they
are BIG starting loads. Thus you load the generator with the
FR regulator, it gulps and slows down, lowering the frequency,
dropping the regulated output, the generator recovers and raises
its frequency, so the voltage.....
If the FR load is a fraction of the total load; it's usually not
an issue. [And remember, the generator must run the HVAC unless
all you want is an orderly shutdown within a few minutes..]
 Here 'ringing' means sharp spikes in the should-be-smooth
 Within a wide range, say 105-135v in for a 120v model. But
get outside of THAT, and...]