SoT wrote:
[...]

They would need a generator for that.  I thought the idea was for
the system to do a controlled shut-down with the UPS.

Yes.

And they have an oddball UPS with a proprietary 'serial' cable with obscure signalling that they've had to time out on.

Another example of proprietary 'bodges' that throw extra expense onto the customers.

Regards,
Martin