Just how much energy can one of these hold (what is
its rated
 voltage?) 
 I have no idea , but 2 volts feels right. 
  What?  That answer is
nonsense.
 Voltage and energy are two very different things, as you must know. 
 
 Yes.  Two questions were asked (one of them parenthetical); the answer
 given answers one of them (but only that one, at least directly).
 It's not all _that_ difficult to work out the energy given the voltage
 and capacitance: joules equals farads times volts squared, which gives 
 
THe energy in a capacitor is actually (1/2)*C*V^2. Basically, you have to
consider charging the capacitor in small increments (dQ at a time) and
effectively do the integral.
-tony