Can't get upsmon service started to monitor (and respond to) remote UPS

You need to edit /etc/nut/upsd.users (on buffalo.local in your case) and define a upsmon user, like so:

password = somerandompassword
upsmon master

Then in /etc/nut/upsmon.conf (on the "monitoring" machine) set the MONITOR line to

MONITOR [email protected] 1 yourupsmonuser somerandompassword master

With a systemctl enable nut-monitor that should be it. Just watch out for SystemD starting up your upsmon instance before the network is available - it will fail to assert MINSUPPLIES 1 and trigger the shutdown of your computer before you're able to edit /etc/nut/upsmon.conf to increase the DEADTIME. Cue lots of "fun".