Capacitor units(Farad)

The unit of capacitance is a farad. A one farad capacitor can store one coulomb  of charge at
1 volt. A coulomb is 6.25e18 (6.25 * 10^18, or 6.25 billion billion) electrons. One amp represents a
rate of electron flow of 1 coulomb of electrons per second, so a 1-farad capacitor can hold 1 ampsecond
of electrons at 1 volt.

A 1-farad capacitor would typically be pretty big. It might be as big as a can of tuna or a 1-liter soda
bottle, depending on the voltage it can handle. So we typically see capacitors measured in
microfarads (millionths of a farad).

To get an idea of farad is,

• A typical alkaline AA battery holds about 2.8 amp-hours.

• That means that a AA battery can produce 2.8 amps for an hour at 1.5 volts (about 4.2 watthours -- a AA battery can light a 4-watt bulb for a little more than an hour).

• Let's call it 1 volt to make the math easier. To store one AA battery's energy in a capacitor, you would need 3,600 * 2.8 = 10,080 farads to hold it, because an amp-hour is 3,600 amp-seconds.

       If it takes something the size of a can of tuna to hold a farad, then 10,080 farads is going to take up a LOT more space than a single AA battery! Obviously, it is impractical to use capacitors to store any significant amount of power unless you do it at a high voltage.