Originally Posted by Branko D

When calculating calories based on power meter data, no - conventional wisdom has it that cyclists don't actually differ all that much (the range last I looked at studies - admittedly with trained cyclists - was about is +- 10%) when it comes to turning calories into watts. Of course your basal metabolic rate differs based on age, sex, weight, height and so on but taking that into account is a bit iffy - you'd have expended (a good part of) that anyway just being alive.

Generally, 200W (which on a road bike on flat terrain with no wind is good for about 31-35 km/hr depending on how aerodynamic you are and how efficient the bike is) requires about 720 kcal plus whatever your basic metabolic rate is (which depends on weight, sex, etc) comes out to let's say about 800 to get a nice round number, so 430 kcal for going hard for half a hour is entirely feasible (or more, or less).

As long as you're watchful what and how much of it you put in your mouth, cycling is going to help.