As a general point, I've found the thinner I am the more tired I feel on the fast, and also closer to fainting when rising from lying down to standing. My first fast of 26 days I began at least 60 pounds overweight, and I had more energy than the latest one, where I was much closer to my ideal weight.
Of course there are so many factors which will affect one's energy levels- toxicity, stress, environment, emotional state, cold weather and so on. Personally, I haven't found that energy levels increase during a fast, nor do I think it's a useful yardstick of one's progress. But it's probably quite nice if it happens.