Currently they're rated to cool ~5W of heat at a cost of 1W with the AirJet, or ~10W of heat at a cost of 1.75W with the AirJet Pro. However, they apparently cause a degree of passive cooling with how they work and their ability to blast out more of the heat from the device overall, leading to the AirJet Pro being capable of cooling something like a 15W Steam Deck:
How does this compare to the power draw of mobile fan systems? I've never been able to get a straight answer on this.
And yes, fans are of course cheaper and still capable of cooling these devices. They're also bulkier and several times louder. But they'll absolutely be the norm for years to come. I hope to see solid-state cooling in the Switch 3, with exactly the sorts of improvements in cooling and energy efficiency you describe.