Q:

How to calculate DC power consumption of a device

I've got an old car radio that I've mounted in my backyard, with some speakers. Right now it's running off an old battery I have, with a solar panel that trickle charges it during the day. I want to know, how many watts/volts it's using and what the solar panel is producing. The goal, would be to add another panel, so it can supplement the power more.. because I'm thinking about expanding it. 
I've got a basic volt meter. If I have to buy something I will. I'm more cirious to learn about it then the 

A:

Watts = Volts x Amps
So, if you measure the amps flowing through the circuit, then multiply it by the voltage across the circuit, you will have the number of watts the system is using.
Switch your volt meter to a suitable voltage range and measure the voltage across the battery (should be somewhere around 12V).
Switch your volt meter to measure amps (if it can - if not get one that will) and connect it in between the battery and the stereo:
  ----(meter)--------
  |                 |
  +                 |
Battery           Stereo
  -                 |
  |                 |
  -------------------

Multiply the two readings, and voila!

