Precisely determine the actual consumed watt of the device

Thread Starter

abdulbadii

Joined Aug 30, 2017
36
What is the actual consumed watt of the device stating a value of input watt and a value of output watt which we often see it ?
 

Yaakov

Joined Jan 27, 2019
3,523
It's hard to understand the exact meaning of your question. Could you give a specific example of a device you want to know about?

Power in W is just current in A multiple by voltage in V. If you know how much current is flowing at what voltage simple arithmetic tells you power.

But there are other interpretations of what you mean, so, an example will help.
 

Papabravo

Joined Feb 24, 2006
16,832
The answer is just that simple for devices that consume power. It is only a bit more complicated for devices that transform power. In that case the governing concept is: (drum roll....wait for it)

Power out will always be less than power in. Sometimes it will be much less.

Figuring out the ratio of power out to power in is seldom a matter of calculation, but rather a result of measurement. You can estimate the ratio or use an estimate as a goal to be reached, but too many factors affect the result to make calculation an easy task except in the simplest of cases.
 

rsjsouza

Joined Apr 21, 2014
335
Power out will always be less than power in. Sometimes it will be much less.
But... But... But... This has to be wrong! My 2800W PMPO amplifier says it consumes only 60W on its 120VAC input! :D

To the OP: @Papabravo is correct and my joke above is the illustration of what is commonly seen in the marketplace: the output power (at the loudspeaker, for example) is the supply power (in the outlet) subtracted from the losses of the equipment - and can never be higher than that.
So, to answer your question: the power consumed by the equipment is the supply power, usually shown in a sticker or engraved in the back of an equipment.

I used an amplifier as an example, but another common example is a microwave oven: at 800W oven usually refers to the RF output power, but the supply power is significantly larger (mine used to be 1100W) - again, this is typically shown in a sticker in the back or the interior of the equipment.

There are many other considerations for measurement, such as RMS or instantaneous power, etc...
https://www.retromanufacturing.com/blogs/news/watts-rms-vs-peak-max
 
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rsjsouza

Joined Apr 21, 2014
335
Indeed it is. And it is so commonplace that, during my university years, I had to convince even graduated engineers of how fallacious and misleading this was.
 

crutschow

Joined Mar 14, 2008
27,759
It seems like at least once a week we have a new poster here insisting they have found a magical way to get more power out of a device then they put in (but, of course, the details are secret).
 

Papabravo

Joined Feb 24, 2006
16,832
It seems like at least once a week we have a new poster here insisting they have found a magical way to get more power out of a device then they put in (but, of course, the details are secret).
Of course, we must all sign that Ironclad NDA (there is no such thing according to the courts) in order to be blessed with the details. Thanks, but time for lunch.
 

Yaakov

Joined Jan 27, 2019
3,523
It seems like at least once a week we have a new poster here insisting they have found a magical way to get more power out of a device then they put in (but, of course, the details are secret).
The working technique is to have other people put power in, violating the laws of the land is possible unlike the laws of physics.
 

Papabravo

Joined Feb 24, 2006
16,832
powerout/powerin = efficiency; Is power factor a consideration?
In DC-DC conversion schemes it is usually not a consideration. Power factor usually shows up when there is an AC source with a reactive load. This can be identified by noting the phase difference between the voltage waveform and the current waveform. The definition is in fact the cosine of the phase difference. For a zero phase difference the cosine of that is 1 and it represents a pure resistive load. In DC power supplies you are mostly dealing with resistive loads.

There is one possible place where it might apply and that is offline forward and flyback converters.
 
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