The Back-EMF generated by an inductor in Volts is: \$V = -L\dfrac{di}{dt}\$
Where \$L\$ is the self-inductance and \$\dfrac{di}{dt}\$ the rate of current change.
Now, let's say I want to store those Volts on a capacitor \$(F, V_0)\$ where \$F\$ is the capacitor capacity in Farads and \$V_0\$ is the initial voltage on the capacitor.
How can I find out the final voltage on the capacitor? How much energy stored on the Back-EMF field?