Looking for a capacitor capable of driving a 12V / 7A valve for ~4 seconds?
Q = C * V
Charge stored in Coulombs is Q
Rated capacitance in Farads is C
Voltage across the plates in Volts is V

Divide both sides by a period of time and interpret V as the change in Voltage across the plates over that period of time and...
Q / t = I = C * V / t
Current is I in Amps if the is time in seconds. 

More precisely, and since things vary continuously in the real world, I = C * dV/dt

The time derivative of Voltage is dV/dt.

Rearrange these with the usual algebra to answer your question. 

C = I * t / V = 7A * 4s / (13V - 7V)... as a coarse estimate.