How do I choose the right size capacitor?

Multiply 0.5 times the square of the voltage. Call this result x. By continuing with the example, you have 0.5 times 11.5 volts for 11.5 volts, or 66.1 square volts for x. Divide the need for starting energy, into joules, of the engine for x to reach the required capacitor dimensions in the farads.