A couple of questions about how efficiency is calculated

[Special K]

I was reading this article here about power supply efficiency:

link

It says:

The efficiency of a power supply is determined by taking the DC output of a power supply and dividing it by the AC input.

First of all, the DC output of a power supply only refers to the voltage - the current will vary with time depending on what you are using your PC for. Additionally, I'm not sure how power factor plays into this - the voltage will ideally be a constant value, but the current will be some random waveform depending on your PC's current usage. Are the voltage and current in phase or not, or does it matter in this case? What type of power (true, reactive, apparent) is being measured on the DC side when it comes to measuring a power supply's efficiency?

Similarly for the AC side, what type of power is being measured when a power supply's efficiency is calculated?

Finally, let's say I have a meter like the https://www.wattsupmeters.com/...hp?pn=0&wai=370&spec=4">Watts Up Pro</a> or the Kill A Watt EZ. Both of these meters claim to measure active power (also called real power) in W. The sample rate of the Watts Up Pro is 1 sample/s. Let's say I plug this meter in between my computer's PSU and the wall outlet. It will display some time-varying power reading in W. I'm not sure if it's an instantaneous power reading or some RMS power value taken over a short interval. That tells me how much active power is being drawn from the wall. Now let's say I would like to know approximately how much power is being used by my PC. I have the PSU's efficiency graph: link.

The equation for efficiency is DC output power/AC input power. Here, I have measured the AC input power but I don't know the DC output power. I do have the efficiency graph, however. Can I use the following iterative approach to find out approximately how much DC power is being drawn by my PC:

1. assume some value for the efficiency (use the chart to make an educated guess)
2. using the assumed value for efficiency, calculate the resulting DC power using the efficiency equation
3. divide the DC power number from 2 by the max rated capacity of the PSU; now check the efficiency curve provided by the PSU manufacturer; if the efficiency number matches up with the one you guessed, then you are done; if not, then use the efficiency number you saw on the graph as your new guess and go back to step 1.

Would that work?

 

[F1shF4t]

Originally posted by: Special K
I was reading this article here about power supply efficiency:

link

It says:

The efficiency of a power supply is determined by taking the DC output of a power supply and dividing it by the AC input.

First of all, the DC output of a power supply only refers to the voltage - the current will vary with time depending on what you are using your PC for. Additionally, I'm not sure how power factor plays into this - the voltage will ideally be a constant value, but the current will be some random waveform depending on your PC's current usage. Are the voltage and current in phase or not, or does it matter in this case? What type of power (true, reactive, apparent) is being measured on the DC side when it comes to measuring a power supply's efficiency?

On the DC side power is always real. Since both voltage and current are not oscilating values (going positive then negative) they are always in phase.
You are right about the current draw being different based on different loads, this is why its important to measure psu efficiency at different load levels.

Similarly for the AC side, what type of power is being measured when a power supply's efficiency is calculated?


For efficiency real power is only important. Net reactive power change over a full cycle is zero.


Finally, let's say I have a meter like the https://www.wattsupmeters.com/...hp?pn=0&wai=370&spec=4">Watts Up Pro</a> or the Kill A Watt EZ. Both of these meters claim to measure active power (also called real power) in W. The sample rate of the Watts Up Pro is 1 sample/s. Let's say I plug this meter in between my computer's PSU and the wall outlet. It will display some time-varying power reading in W. I'm not sure if it's an instantaneous power reading or some RMS power value taken over a short interval. That tells me how much active power is being drawn from the wall. Now let's say I would like to know approximately how much power is being used by my PC. I have the PSU's efficiency graph: link.


Don't you mean average. It will probably be an average of some small period to elliminate instantaneous spikes.


The equation for efficiency is DC output power/AC input power. Here, I have measured the AC input power but I don't know the DC output power. I do have the efficiency graph, however. Can I use the following iterative approach to find out approximately how much DC power is being drawn by my PC:

1. assume some value for the efficiency (use the chart to make an educated guess)
2. using the assumed value for efficiency, calculate the resulting DC power using the efficiency equation
3. divide the DC power number from 2 by the max rated capacity of the PSU; now check the efficiency curve provided by the PSU manufacturer; if the efficiency number matches up with the one you guessed, then you are done; if not, then use the efficiency number you saw on the graph as your new guess and go back to step 1.

Would that work?

This gives you the % utilisation of psu not efficiency. The psu will only consume the power thats needed.
DC power/AC Power * 100 = efficiency
The data you have is AC power and presumed efficiency, all you can do is calculate the presumed DC power.
Iterative method is not appropriate here.

 

[Special K]

Originally posted by: Dark Cupcake

Originally posted by: Special K

Finally, let's say I have a meter like the https://www.wattsupmeters.com/...hp?pn=0&wai=370&spec=4">Watts Up Pro</a> or the Kill A Watt EZ. Both of these meters claim to measure active power (also called real power) in W. The sample rate of the Watts Up Pro is 1 sample/s. Let's say I plug this meter in between my computer's PSU and the wall outlet. It will display some time-varying power reading in W. I'm not sure if it's an instantaneous power reading or some RMS power value taken over a short interval. That tells me how much active power is being drawn from the wall. Now let's say I would like to know approximately how much power is being used by my PC. I have the PSU's efficiency graph: link.


Don't you mean average. It will probably be an average of some small period to elliminate instantaneous spikes.


The equation for efficiency is DC output power/AC input power. Here, I have measured the AC input power but I don't know the DC output power. I do have the efficiency graph, however. Can I use the following iterative approach to find out approximately how much DC power is being drawn by my PC:

1. assume some value for the efficiency (use the chart to make an educated guess)
2. using the assumed value for efficiency, calculate the resulting DC power using the efficiency equation
3. divide the DC power number from 2 by the max rated capacity of the PSU; now check the efficiency curve provided by the PSU manufacturer; if the efficiency number matches up with the one you guessed, then you are done; if not, then use the efficiency number you saw on the graph as your new guess and go back to step 1.

Would that work?

This gives you the % utilisation of psu not efficiency. The psu will only consume the power thats needed.
DC power/AC Power * 100 = efficiency
The data you have is AC power and presumed efficiency, all you can do is calculate the presumed DC power.
Iterative method is not appropriate here.

Right, but consider the equation:

efficiency = DC output power / AC input power

We know the AC input power - it was measured. If we assume some value for the efficiency, we can then calculate a DC output power using the efficiency equation. We can then check our answer by taking our calculated DC output power, dividing by the PSU capacity, and then using the PSU manufacturer's efficiency graph to see what efficiency corresponds to our calculated/estimated value of DC output power. If it's off, then we make a better guess for the efficiency and try again.

 

[HOOfan 1]

Kill-A-Watt meters are not always accurate. The only way to catch accuracy problems is to know the DC output of your PSU, if the efficiency seems way too high then the Kill-A-Watt is obviously not reading AC draw accurately.

So to know efficiency, you really need to know the DC output of your PSU.....that would require using a load test with a specific load, or using an ammeter to measure current and multimeter to measure voltage.

Not all PSUs will fit the same efficiency curve anyway. It especially matters at what temperature and AC voltage input you are measuring from.

 

[F1shF4t]

Originally posted by: Special K
Right, but consider the equation:

efficiency = DC output power / AC input power

We know the AC input power - it was measured. If we assume some value for the efficiency, we can then calculate a DC output power using the efficiency equation. We can then check our answer by taking our calculated DC output power, dividing by the PSU capacity, and then using the PSU manufacturer's efficiency graph to see what efficiency corresponds to our calculated/estimated value of DC output power. If it's off, then we make a better guess for the efficiency and try again.

Ok i see what you mean 😱, using the psu utilisation to lookup the efficiency off the graph.
Got a bit confused there though you were using the utilisation number to compare to efficiency 😛.

I think HOOfan 1 pretty much nailed it. The graphs provided don't reflect the psu operation in all cases. Temps, AC voltage as well as variation in components and manufacturing will affect psu efficiency. If you are just after a very rought estimate then it'll work.