I just read about Screenlets.
I was wondering if using Screenlets will affect the battery life of my notebook?
Well, anything that causes disk io or increased CPU cycles is particularly able to affect your battery life, unless your machine is configured to run full steam regardless. I think how much your battery life will be affected would range from hardly affected all the way to noticeably.
My guess is if you use them within sanity, as in run a few modest ones, you won't notice a difference.
So to answer the question more directly: "Yes, using screenlets will affect your battery life, with some exceptions"
I believe this answers your question sufficiently, right?