Sorry to resurrect an old thread, but since it's toward the top of the list, I won't feel too bad about it. Just wanted to share my experience.
I have a LOT more energy being ZC. Previously, I worked out constantly, but I hated running, hated anything more than super gentle yoga. Even yoga exhausted me. I had no energy. The only time I've had less energy was FIRST going ZC. First transitioning is hell. I have a 3-year-old little boy and the first several days to week of trying to keep up with him and be a good parent were hideous. I just wanted to sit, sleep, stay in bed, etc. The only reason I stuck with it is because of the EXTREME relief when it came to my digestive system. I had a flat stomach and no pain (from looking 9 months pregnant and constant agony and digestive distress for years) within two days of starting. After about a week, I suddenly had more energy than I've had in years and now play with my 3-year-old more than I ever have. My house is also immaculate! It's seriously like I'm on speed or something, I get so much done in a day. Mind you, it probably only took a week because I was already LC/paleo prior to going ZC (the bloating came from IBS, which was the result of a condition called fructose malabsorption).
Since beginning, I have "cheated" a few times. I've thought eating one item would be ok, like part of a potato, or just recently, a couple blocks of Kerrygold grassfed cheese. Now whenever I ingest too many carbs of any type, I'd say anything over 5g (a limit I sometimes approach due to egg consumption), my energy is suddenly crap again and I have to readjust. Now it generally only takes a few days to get back to what I'm guessing is just full on ketosis, but then again, I never go and carb binge or anything.
So I don't know if the issue is getting too many carbs in other areas... I know some ZCers use heavy cream, eat cheese, tons of eggs, liver regularly, etc... but it can add up. And especially if you occasionally have a carb source. Otherwise, you should adjust.
If it's truly not that, then I'd imagine it's not enough fat. If I'm feeling tired, it's always one of three things: 1.) I haven't slept enough (surprise, surprise), 2.) I haven't had enough fat, 3.) I haven't had enough water. Lots of water and lots of fat and I'm good to go. I often have so much energy that I have to go bike for about an hour or run just to burn some of it. I also lift weights several times a week and sit down maybe once a day, at night, for like half an hour. Soooo much energy.