On Fri, 5 Feb 2016 01:39:54 +0000, Brian Gregory
wrote:

On 30/11/2015 17:55, Danny D. wrote:
Scott Lurndal wrote, on Mon, 30 Nov 2015 17:38:04 +0000:

Q1: Since 6.8A times 5VDC is only 34Watts (not 40 Watts), how
can they very clearly label it as a 40Watt device?

http://www.powerstream.com/VA-Watts.htm

Here is a better (bigger) picture of the unit:
https://i.imgur.com/8bl7ypU.jpg
And a better (bigger) picture of the package:
https://i.imgur.com/30qxupn.jpg

I thank you for the reference, but, after reading it, I don't
see how that article helps explain the *output* wattage
descrepancy between 6.8A x 5VDC and the stated output spec
of 40Watts.

That article explains how to convert Volt-Amps to Watts.
"How to convert VA to Watts and KVA to Kilowatts"
http://www.powerstream.com/VA-Watts.htm

The input voltage is AC so roughly the input wattage is:
110VAC RMS times 800mA (max) which is roughly 88VA

But it's the OUTPUT wattage that is stated to be 40Watts.
6.8Amps x 5.0VDC is only 34 Watts

No, I don't think it necessarily is, it just rather vaguely says
"Maximum power: 40W" on a separate line without any real clue as to what
it refers to.


Hence, a rather large 15% descrepancy (6 watts less than
40 watts).

I'm sure my math on the output wattage is wrong because
the package can't be wrong (that would be illegal), so,

In my experience many Chinese manufacturers wouldn't care about the
legality of giving a slightly wrong number. Many lie outright about
meeting safety standards.


I don't see where my math went wrong yet though.

So, any advice is welcome and I will check it out.

It's what you call (in)efficiency.