Yes you are low on energy because your body is used to get food / energy regularly inserted and is not yet adapted to make good use of the stored bodyfat energy, this is individual and can take some time, days, maybe even weeks! I think best is to go full keto for some time and give the body a chance to get fat adapted.
electrolytes, esp salt, see sidebar at r/fasting. hydration is a balance of sodium and water, you need the sodium to hang onto the water and prevent flushing out other electrolytes. low sodium > fatigue, confusion, headache