As I said you have to make an assumption about your metabolic efficiency in turning your food/stored energy into mechanical work. There are about 4.2 kJ in a kcal (food calorie), so if we were 100% efficient, we would only need to eat about .24 calories for every kJ our power meter shows us. But we are not that efficient. Testing has shown that most people are between 18 to 24% efficient. So if you assume 24% efficiency, then 1kJ would be 1kcal, but if you took the other end of that, you are looking at a factor around 1.15, and that's a big difference.