If you're shooting a shield, the power damage/drain applied is correct. It should be 10 times more than the listed damage (and it is that) according to the block config.

When your shots penetrate however, the power drain is <penetration depth> times higher. So if I shoot something that has penetration depth 28 and it doesn't get blocked/stopped somewhere, my power drain hit would be listed damage * 10 * 28.

This applies for cannons and beams, although beams don't penetrate on armor blocks so you don't notice it on that unless you hit systems.

How to reproduce:

1. use any ship with some penetration and a cannon + emp effect
2. shoot at a target and compare the actual power drain versus what it should be (should always be 10 times the listed damage)