Worries About Energy Loss

Ok here is my question for all of you veteran fasters!! I work at JcPennys and energy is something i need plenty of to make sure i dont get in trouble for not pulling my weight around the store. I was wanting to start a 7 maybe 10 day Water Fast but was wondering if i would have a decrease in energy since i wont be eating anything... i was also wondering if i weigh between 137-143 how much weight would you think is a resonable goal for me to lose in 7-10 days (if i can make it the extra 3 days to make a 10 day fast!!!!)Please post your comments i need as much help and support as i can get! Thanks everyone and happy fasting!! xoxo Tweet