Question About Switching Power Supply’s Wattage Generates Novel Comments Exchange

How efficient is a typical 5V AC-to-DC power supply? I’m digging through my box of assorted power supplies from past projects, looking for something appropriate for a large LED matrix, and noticed the one pictured above. 110V 0.8A input – that’s 88 watts. 5V 4A output – that’s 20 watts. Is the power supply really only 22.7% efficient, or is my reasoning flawed somehow? I would have expected a switching power supply like this one to have much better efficiency than that. A quick search for information on “level IV power supplies” suggests they must be 85%+ efficient, so something doesn’t add up.

Big Mess o’ Wires posted the above blog about a Phihong DC switching power supply they had, which generated a novel exchange in the comments about root mean square, power factor, volt-amps, and other topics of intrigue. It’s not often we get to say it but, Read the Comments!