this post says:
Per the 13th Edition Telecommunications Distribution Methods Manual Chapter 9: Power Distribution
one watt is equal to 3.412 BTU/hr
So to determine how much heat you need to dissipate you take the total number of watts the equipment will consume and multiply that by 3.412.
the power consumption of 23W at 12V will not differ much with that at 24V, because the current will be less.
So unless you are putting hundred switches in the same cabinet a difference of 1W in your calculation wil not be alarming.