Register a batch file with shutdown command in /etc/nut/upsmon.conf
SHUTDOWNCMD "/home/user/batch/shutdownall"
Install NUT in all linux clients and connect them to the NUT server
Make sure you can ssh as root into every client which needs to be shutdown manually with keypair, and without entering any password. Add a line for each client to this batch file:
ssh root@client "shutdown -h +0"
Shutdown manuall
Create a batch file with shutdown commands for each client: