Low-voltage at any meaningful distance is extremely lossy. Unless he's using big expensive wires, he's going to have a lot of voltage drop on any load beyond a small gadget. Even just pulling, say, 50 watts out of a 12-volt socket with a 20-foot run of 12 AWG behind it gives you a voltage drop of over 2% at the socket. At 120V/12AWG, the drop would be 0.22%. You'd have to use ~2AWG to match that. That's a lot of copper.