Capacitor time formulas (for noobs)

Hi everyone. Let me make it VERY CLEAR that I am not planning on using a capacitor as a tank circuit. I'm only writing this because I'm reading my electronics book, and ran across this. I thought I would point this out for the n00bs.

Time = Farads * Ohms

That means, that how many ohms your computer provides, times the farads of the capacitor is how many seconds it will last.

Volts = Amps * Ohms

therefore Ohms = Volts / Amps

therefore Time = Farads * (Volts / Amps)

Volts is usually 12.

So, for my 120 watt computer, I can figure out the amps by dividing by the voltage.

120 watts / 12 volts = 10 amps. I can plug that in my formula no problem.

Time = Farads * (12 / 10)

However, I need to know either

a) how long the capacitor I have will last

or

b) how big a capacitor is needed to last x seconds.

I said, I need a capacitor that can last 10 seconds.

10 = Farads * (12/10)

10/(12/10) = Farads

8.3333 Farads are required for a "normal" car startup. Do you have any 8 farad capacitors laying around?