Rather than answering "How" - I'd probably be better off explaining why the statistics don't exist, and why you shouldn't actually worry about this.
The simple fact is, compared to other components, Its probably negligible. There's no up to date statistics I can find with a quick search, but there's a comparison with older card models that shows at most a 15 watt difference between a onboard and discrete card and about 5w between cards adn the wall. Though, there were also differences in processor usage, and this reflects power usage of the system rather than the sound cards themselves
Theoretically you would use between 10W (for a half height x1 slot) and 25 watts (for a full size x1 or bigger card) maximum for a PCIe card and 5W for a USB device I'm referencing these numbers from the wikipedia - I used the pci e page, and a worst case scenario of 1A for the USB device, since they typically run from 0.5 to 0.9A.
In comparison, my desktop speakers use about 12W for a 2 speaker and no subwoofer set up, and the amplifiers for your speakers probably are going to use more, even if they're class D or T rather than the class A, B or AB designs.
That is to say, no one bothers with sound card power usage because it dosen't matter.
If you really wanted to minimise - consider this, a great headphone amplifier chip of the sort you would find in a high end sound card is only 1.5w, and I can't find overall power requirements for a DAC chip in a casual search (but quite a few good ones are usb powered, so we can assume a maximum of 5w) and that's likely one of the more power hungry 'add on' parts that would differenciate a sound card.
Anything with a heatsink is likely going to draw more power (and is probably a good, strange rule of thumb in this case), so all other specs being equal, go for the one that looks simpler, and has just the features you need. It'll end up being cheaper, and probably runs with less heat.