A bullet's kinetic energy is a function of its velocity. The formula is K.E. = 1/2 m times v-square (I dunno how to do superscripts.) m is mass (in Poundals; a Poundal is the weight in pounds divided by the acceleration of gravity, or 32.16), v is velocity in feet per second.
Anyway, to compare any two cartridges, take the ratio of the square of the velocities; and the ratio of the bullet weights, and you get an energy comparison.
So: The typical AK bullet is 139 grains. The typical .308 bullet is 150.
The muzzle velocity of an AK is around 2,300 ft/sec. For a .308, around 2,800 ft/sec.
Divide 150 by 139 and hold that number. Drop the zeros, and multiply 28 by itself and divide it by 23 multiplied by itself.
Thus, 150/139 = 1.079. 28x28/23x23 = 1.482.
1.079 x 1.482 = 1.599. So the .308 has 60% more muzzle energy than an AK.
In the future, the math will be left as an exercise for the student...