2400 kcal can be a useful figure if you don't want to do the full calculations, but it is assuming an average build male who is moderately active - for example, a warehouse job.
When completely inactive, the body is using much less than that. I'm a 170 cm tall, 63 kg female. My basal metabolism is 1350 kcal per day - this is 56 kcal an hour. That's what I am using when asleep/lying down. For a (bigger) male, this would be a bit higher. As you lose weight, this figure will drop a little.
If I do an hour of cycling, I can add 459 (515-56) kcal to this figure.
If I do 10 mins of walking around, I can add to the figure.
If I do 4 hours of sitting watching TV, I can add (a small amount) to this figure.
The calculations are a bit laborious, which is why the 2400 figure is used. The danger of the 2400 figure is that a lot of people reward themselves with food after going to the gym, thinking that they have worked it off by adding the exercise figure to the 2400.