I did some searching, and it appears that once various fuels are actually metabolized, the percentage of available released is similar for both Fat and Carbs (about half.) But, the real issue is not what percentage is released, but what is wasted and how the energy is used. A "High Protein/Low Carb" diet is believed to be less efficient by some because it releases more energy as heat than a High Carb Diet. One other theory is that when ketone levels begin to rise too high, the body has to excrete them to keep its pH within an acceptable range. Thus, the body excretes fuels (Ketones) unused.

Unless you're a Diabetic, you don't excrete Glucose unused. Glucose is very efficient in that manner. If its not used its either converted back to Glucose/Glycogen, or converted to Fat, and stored on the body. Even in an Anaerobic Metabolism, when Glucose yields very little ATP (Energy) per unit of Glucose, it produces a byproduct (Lactate,) which the body can further Metabolized to get more ATP (Energy.) Lactate can also be converted back to Glucose [with very little loss of Energy] later on, and used again.

http://web.indstate.edu/thcme/mwkin...metabolism.html

Protein is unique in that depending on the Amino Acid composition, it may be metabolized more like Carbs or more like Fat. Amino Acids come in 3 types: Ketogenic, Glucogenic, and Keto/Glucogenic. In some instances, Amino Acids can be directly metabolized for energy.

The Ketogenic Amino Acids (Lysine and Leucine) CANNOT be converted the Glucose. They are metabolized to Acetyl CoA, the same stuff Fatty Acids are Metabolized to. From there, they are metabolized the same as Fatty Acids, including the possibility of being converted to Ketone Bodies.

The Keto/Glucogenic Amino Acids (Isoleucine, Phenylalanine, Threonine, Tryptophan, and Tyrosine) can be metabolized to either Acetyl CoA, Pyruvate, TCA Intermediates, or a combination thereof. Pyruvate and TCA Intermediates can be either metabolized for energy or converted to Glucose via Glucogenesis.

The Glucogenic Amino Acids (Glutamate, Aspartate, Alanine, Ornithine, Proline, Serine, Cysteine, Methionine, and Glycine) CANNOT be directly metabolized to Acetyl CoA, and thus don't yield Ketone Bodies. They are metabolized to Pyruvate or TCA Intermediates, which may be metabolized for energy or converted to Glucose.