0 Three Dollar Wattmeter

I've got a watt meter, but if you're wanting to measure your power system on the cheap, here's a link to the classic $3 Wattmeter based on the ubiquitous Harbor Freight digital voltmeter.

Basic Idea:  add 14 gauge wires, attach to connectors of your choice, and wire in a 1K-10K resistor to protect the circuitry when flipping through the millivolts settings.

The stock wires are good for up to 10A, and with the replacement wires it's good up to 20A.

Jackerbes has some nice construction photos. critterhuner has some here.

Here's a wiring diagram.

You could run two of these in series, one measuring voltage and one measuring amperage for six dollars.

And Lucien has a nice writeup on cheap power shunts that will extend the current measuring range up to 55A continuous and 100A burst.

For those interested in learning more about watt meters, check out my link above.  Here's the manual for the well known Watt's Up unit.