Due to some serious power outages, I had to install an UPS at the Office of one client. It is an rather small setup: One low-power Ubuntu Server, one Laptop with one TFT Screen, one i3 Desktop with two TFT Screens, one network switch. All in all, about 400VA. I had an old APC Smart UPS 750 VA at hand and used it.
Idea was to connect the UPS via USB directly to the Server and hook Laptop and Desktop to that Server via Network. As soon as the Server found that the whole Powergrid went offline, all pcs should shutdown automatically: Enter apcupsd.

Power installation:
Connect the UPS input to the power grid, connect the UPS out to your PCs. NEVER CONNECT ANY LASER PRINTER TO THAT OUTPUT!
Power up the UPS.

Server installation:
Connect the UPS USB Port to the Server.
Install apcupsd:
sudo apt-get install apcupsd
Configure apcupsd:
sudo vi /etc/apcupsd/apcupsd.conf
In my case I configured that settings:

UPSNAME blaUPS # How you want to name your ups
UPSCABLE smart # in my case, it is a smart cable
UPSTYPE usb # on usb
POLLTIME 60 # poll ups every 60 seconds
ONBATTERYDELAY 10 # delay alarm for 10 seconds
BATTERYLEVEL 10 # on less than 10 percent battery level shutdown server
MINUTES 3 # on less than 3 minutes battery runtime shutdown server
NETSERVER on # activate network server
NISIP 0.0.0.0 # allow access from all nics
NISPORT 3551 # default port for network server

Allow port 3551, tcp through iptables!

Restart apcupsd:
sudo service apcupsd restart

Give status of current apcupsd session:
sudo service apcupsd status

Client installation on Windows:
Download latest version for Windows (i.e. winapcupsd-3.14.13.exe), you only need apcupsd Service and Tray Applet.
Leave everything on default on setup and configure apcupsd.conf

UPSNAME blaUPS # How you want to name your ups
UPSCABLE ether # network to server
UPSTYPE net # on network
DEVICE IP:3551 # for IP, enter the IP of the server
POLLTIME 15 # poll ups every 15 seconds
ONBATTERYDELAY 10 # delay alarm for 10 seconds
BATTERYLEVEL 20 # on less than 20 percent battery level shutdown client
MINUTES 3 # on less than 3 minutes battery runtime shutdown client
NETSERVER on # activate network server
NISIP 127.0.0.1 # allow access only from localhost

And thats it 🙂