[nimmen]

5V * 5A = 25W, no PSU is 100% effective so 27W is reasonable requirement

[marcosdumay]

You don't specify a PSU by its energy consumption. You specify it by output.

Also, that 92% efficiency, is believable, but a bit high for a 5V 5A PSU (this is a difficult combination). I would expect any such unity to be marketed as high-efficiency.

[ralphm]

I expect it is 27W to support 9V 3V, not because of tolerances per se. (Source https://www.sparkfun.com/products/23583)