Here's a very useful link which allows you to instantly work out the Kinetic Energy (power) of your airgun, without having to do the maths calculation yourself.

It also gives a guide to the weights of many popular pellets, though weighing your own pellets is highly recommended, as pellet makers quite often change the specification of the pellets they make, but rarely tell anyone, so the quoted weights my no longer be correct.

http://extorian.co.uk/shooting/pellets.html