The only way to accurately match calibers for "power" is to consider the weight of the bullet, the speed and the caliber.
For instance, a big bullet that moves slow, will have different power factor than a small bullet that moves fast.
The power factor is used to measure all of the effects for the various clubs that compete.
http://www.eskimo.com/~jbm/calculations/pf/pf.html
Here is a site that you can plug those factors into and it will calculate the Power factor for you, from a scale of 1 to 1000. It works for both pistols and rifles.
Just plug in the speed, weight of the bullet and the caliber and hit calculate and it will give you a number. Take the time to play with it and see how the various calibers stack up against each other.
You'll soon notice that pretty much anything below .38 special becomes pretty marginal for self defense.
As an example, a 10mm with a 180 grain bullet moving at 1350FPS has a rating of
243, verses a 9MM shooting a 124 grain at 1250FPS which has a factor of
155.
Lot of difference there...