Possibly counterintuitively, adding time at low intensity is way more effective at burning calories than a harder workout. Hard workouts are relatively inefficient at burning calories (unless you have limited time).
A typical tough TrainerRoad interval session might be something like an hour at 0.85 IF – so, the NP is 85% of your FTP. But the average power will run more like 70-75% of your FTP. An hour at 75% is easier but burns the same calories. 1.5 hours at 65% burns the same as a 1-hour all-out effort!