You have to open the lines somewhere. Since it's likely that you have multiple wires for each voltage it's probably best to do it close to the source where you have hopefully only one line. What's best is if you have a fuse block, because you can simply remove the fuse and place one meter lead on each side of the fuse holder.
First make sure your meter can handle what the maximum is that you may be reading - which is probably going to be less than 10A in your case.
Basically, you want to shut everything off and open the line somewhere. Set your meter to current mode, and if your meter doesn't have autoranging, set the current to the highest level. Clip one meter lead to the bare wire on one side of the break and clip other lead on bare wire on the other side of the break. Turn the system on and record the value. You'll want to do this with each voltage rail.
BTW, current is not the same as power. Power is the product of voltage and current.
<small>[ July 29, 2003, 02:17 PM: Message edited by: RiotoftheBlood ]</small>