On Thu, 02 Mar 2017 08:13:54 -0800, Jeff Liebermann
wrote:

On Thu, 02 Mar 2017 09:58:26 -0500, Bob Engelhardt
wrote:

The biggest surprise was the PS's efficiency, or lack thereof. A
Kill-a-watt on the input showed 200w being used, and 54w output. 25%
more or less. I thought that a SMPS would be way more efficient than that.

I'll assume it's an ATX style power supply.
http://www.corsair.com/en-us/blog/2012/august/80-plus-platinum-what-does-it-mean-and-what-is-the-benefit-to-me
"Intel’s ATX specification only requires that a power supply
is 60% efficient at 50% load. Most decent quality power
supplies made in the last decade are around 70% efficient
at 50% load."

If you're getting 25%, either something is wrong, but I can't tell if
it's your measurements, a defective design, a failed power supply, or
how you're measuring the efficiency. I've made similar measurements
of an ATX power supply using a dummy load on the +5v and +12V lines,
and a Kill-a-watt meter. Typically, I obtained 75% efficiency at
about 50% of full load on a cheap ATX PS.

As MJC mentioned, the heat has to go somewhere. If your efficiency at
at 50% of maximum load (200 watts) is only 25%, then you should have
150 watts of hot air and flames coming out the back end of the PS with
the temp controlled fan spinning furiously. Something is wrong.

Photo of one of my loads:
http://802.11junk.com/jeffl/pics/drivel/slides/dummy-load-inductance.html


I was going to respond to the OP, but Jeff beat me to it. Switching
supplies are notorious for having very poor power factors under light
load. I looked up the specs on the kill-a-watt and it is supposed to
read power factor correctly when seeing a pure sine wave. Of course,
there is no guaranty the current waveform is a pure sine wave. It is
more likely to have spikes all over the place. (That's one of the
reasons switching supplies can cause RF noise.) My guess (and that's
all it is) is your supply is using under well under 100 watts, but
your kill-a-watt is confused by the strange current waveform and says
it is using 200 watts.