Energy is just a calculation measuring mass and velocity.
For handgun rounds at typical handgun velocities (say ~1,250 fps or less), energy numbers do not matter much. Handgun bullets crush the tissue in front of them. A flat nose or expanded hollow point is going to crush more tissue. It doesn't matter if the bullet is moving at 500 fps or 1,000 fps. The way the bullet damages tissue remains the same. "Transferring energy" isn't what stops people. Causing damage to vital organs or blood vessels does. Choose a load that will be most destructive to the insides of your attacker. I believe an FMJ or fragmenting round that penetrates <4" is not as effective as a good expanding JHP load that will penetrate between 10"-16".