Considering that any number trying to show you how much energy you expended is a gross guesstimation anyway it seems pointless. Calories and kilojules are trying to describe the same thing on a different scale.

Power meters will perhaps be more accurate, but not completely. They measure the work you do and express that with some fudge factors to account for energy lost that was not directly applied to moving you and your bike further down the road. Devices with only HR monitors do similar by trying to guess from your HR how hard you are working. Devices that don't have either Power or HR are applying simple formulas to further just guess.

The big thing is just not compare to another. Use the same method consistently and learn when you might need to add or subtract from the energy expenditure of your body.