an APC UPS is connected to the Proxmox VE server locally by USB or COM.
In case of a power failure Proxmox VE will not shutdown when the battery is getting to low.
Simple because there is nothing installed on the server to tell that Proxmox VE should make a clean shutdown for the VM's and then shutdown the server properly.
With apcupsd this can be done.
It is also reachable by browser so you can see the remaining battery and more.
Nearly every APC UPS should work with apcupsd.
In case of a power failure Proxmox VE will not shutdown when the battery is getting to low.
Simple because there is nothing installed on the server to tell that Proxmox VE should make a clean shutdown for the VM's and then shutdown the server properly.
With apcupsd this can be done.
It is also reachable by browser so you can see the remaining battery and more.
Nearly every APC UPS should work with apcupsd.