So I noticed, that my media server, my cable modem (internet), my phone (connected to the cable modem), my Wii, my cooling fans for all of this and my external disks all run of 12 VDC. They are also physically close together, seeing they "live" in the same closet. I figured it might be better for my electricity bill and general tidiness to run them all from a big 12 VDC, 20A supply. That would also give me the option to put a car battery in there and have a poor man's UPS.
My concern is, when I connect them all, I might create unwanted potential difference, i.e. between the ethernet on the media server and die cable modem connected to, well, the cable network. Or phone and cable network. Is that a real concern? What should I measure and check before I do this? Am I missing something?