For the following equations, e=volts, i=amps, r=ohms, p=watts.
e=i*r, p=i*e or p=i^2*r, i=e/r
You should always know e and r, so to find p and i, you just need to manipuate e and r in the above equations. Amps are important, as they indicate how hard your battery is working, watts is important, as it indicates how hard you are cooking your ejuice.