It is as accurate as the generic advertised calorie count reported for a McDonalds chicken burger, compared to actually measuring the calories in a randomly selected McDonalds chicken burger.
@PGM : I completely agree: Optris is no replacement for exact calorimetry. Only if you have two identical objects, with identical heat generation, then you may compare the "calories".
Or simply said: With Optris there are to many free parameters, that you must keep an eye on.