Capacitor confusion

[Chrs]

So I read a page where someone said they had put 18 400v 3900uF capacitors in parallel and that this came out to 5600 joules... Now I'm sure I am doing something wrong but when I did it out I only got 28.08 joules...
I used this website: http://www.rapidtables.com/calc/electric/Volt_to_Joule_Calculator.htm
plugged 400 in for volts and 0.0702 in for coulombs, if someone can let me know what I am doing wrong here please let me know.

Second little question, i know if you put capacitors in parallel you just add the Farads, but if they all have different volts, say one has 20v 3000F, the other has 400v 90uF, what would that end up being in parallel?

 

[hevans1944]

Energy stored in a capacitor is 1/2 CV^2 = (0.5) (18) (3900 x 10^-6) (400)^2 = 5616 joules. Enough to kill you.

Capacitors is parallel add their capacitance. Voltage rating of the parallel combination is that of the lowest voltage rated capacitor, or 20 V in your example. Makes no difference if there are higher voltage rated capacitors in parallel, you can still only apply up to 20 V.
.

 

[Chrs]

Thanks sir, I kinda figured 5.6K joules was enough to kill mostly anyone. So if you put like 100V to a set of capacitors in parallel with that 20V rated one on there would the electricity just arc across the plates and make a circuit? And If that happens does it damage the capacitor? (I guess this kinda depends on the type...) I imagine it would

 

[davenn]

So if you put like 100V to a set of capacitors in parallel with that 20V rated one on there would the electricity just arc across the plates and make a circuit?

most likely worse