They make a model that does both amps and volts but this model just measures amps. You wire the shunt on the negative terminal of whatever circuit you want want to keep an eye on. There are 4 wires that go the shunt; the 2 negative cables , and 2 small wires (need to be a twisted pair) that go to the display. The display also needs small positive and neg wire to power it.
I bought this originally to mount on my house battery bank so at a glance I could see how bad the discharge was and how much the net gain was really going into the bank while running, and at what RPM I needed to run to have a decent charge go to that bank. The diagram below shows how I was going to set it up:
Strategies for Monitoring DC Current
This would be set up as "1" in the above figure.
This lists for about $170, the one that does amps and volts is about $250.
In the end, I decided to go with an even fancier one that keeps track of amp hours as well as a gauge on the positive alternator circuity (so now I can see how much is being produced, where it is going as well as how much I have left....but in the end that ended up costing me 500 when all said and done (gauges, wiring, new cables etc) not to mention about 8 hours of my time. So when I was all finished, I had this one left over, I know if I put it in the box with all my other boat stuff it will never see the light of day, so I am putting it up here for about 1/2 of the list price. For the money it is useful for watching your house bank situation both underway, at idle and at anchor or drift.