The transport of electrons from redox pair NAD/ NADH to the final redox pair 1/2 O/ HO can be summarized as
1/2 O + NADH + H  → HO + NAD
The potential difference between these two redox pairs is 1.14 volt, which is equivalent to -52 kcal/mol or -2600 kJ per 6 mol of O.
When one NADH is oxidized through the electron transfer chain, three ATPs are produced, which is equivalent to 7.3 kcal/mol x 3 = 21.9 kcal/mol.
The conservation of the energy can be calculated by the following formula
Efficiency = (21.9 x 100%) / 52 = 42%
So we can conclude that when NADH is oxidized, about 42% of energy is conserved in the form of three ATPs and the remaining (58%) energy is lost as heat (unless the chemical energy of ATP under physiological conditions was underestimated).