I don't know how big your current load is, but since you're wanting to transition to panniers, may I make a suggestion? Think about getting one of those grocery panniers. Depends on how big/full your backpack is, but going this route has a couple of advantages.

1. Assuming your current pack will fit inside, this should give you an idea of what riding with a single pannier will be like.
2. More versatile- you can stop off at the store on the way home, fill up your pack and put any over-flow in the pannier.
3. Grocery panniers tend to be cheaper.