how would I go about building a a rear carrier mount bike generator?

I am trying to build a device which would mount on the rear carrier of my 20 inch folding bike, which would generate and store electricity, for anything, within reason that can be plugged in (i.e lamp, computer, stereo etc).
I bike about 10-15 miles a day and I would like to generate some power and $ave on my elecricity bill (I already have solar panels) and create some green power.
So I want to create, in essence, a kinetically generated charger to mount on my bike, which, when I get home, I can plug appliances into.
If such a product already exists where can I get it. If I need to make it from scratch what is my first step?