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JB

power, watts, VA and power factors

Have we got any electrical engineers around here?

I've been measuring power consumptions on a lot of the equipment I have around here and have noticed that a lot of it runs at very poor power factors. For example my laptop runs at 34W, but 53VA a power factor of about 0.65.

So if the laptop is using 34W but the grid has to supply 53VA ... where does the other 19W go to?
Jamanda

Dissipated as heat I would imagine.
JB

Yes but where? At the power station, in transmission, in the inductive / reactive load?

and does this mean that a simple reading of power can not be translated into a weight of CO2?
RichardW

The measurement of electrical power that is computed by multiplying volts times amps. In a DC circuit, volt-amps (VA) and watts are the same because DC circuits do not add inductance and capacitance that take away power. In AC devices, volt-amps ratings are higher than watts. For example, in an uninterruptible power supply (UPS), the volt-amps rating is approximately 60% higher than the actual wattage.
JB

Justme wrote:
The measurement of electrical power that is computed by multiplying volts times amps. In a DC circuit, volt-amps (VA) and watts are the same because DC circuits do not add inductance and capacitance that take away power. In AC devices, volt-amps ratings are higher than watts. For example, in an uninterruptible power supply (UPS), the volt-amps rating is approximately 60% higher than the actual wattage.


Well yes I'd got that far in trying to work this out. What I can't find for certain is whether the power factor is just an artifact of the measurement and that phase difference or represents a real power loss. I assume the latter otherwise why would the generators worry about it. Assuming it is a power loss then where is that power lost, in transmission, load, generation? And assuming that is right then I assume that for purposes of calculating environmental impact that VA is more significant that wattage.

(BTW Mrs JB has just asked what I was talking about as she could hear me typing away. When I told her she just said "I'm so glad I asked" Very Happy )
vegplot

Re: power, watts, VA and power factors

JB wrote:
Have we got any electrical engineers around here?

I've been measuring power consumptions on a lot of the equipment I have around here and have noticed that a lot of it runs at very poor power factors. For example my laptop runs at 34W, but 53VA a power factor of about 0.65.

So if the laptop is using 34W but the grid has to supply 53VA ... where does the other 19W go to?


Where are you getting the 53VA figure from?
RichardW

JB wrote:

(BTW Mrs JB has just asked what I was talking about as she could hear me typing away. When I told her she just said "I'm so glad I asked" Very Happy )


I get that A LOT
Lol

vegplot wrote:

Where are you getting the 53VA figure from?


I am guessing he has a power meter that plugs into the socket like mine that shows volts, amps, watts, kwh, hours, VA, PF (power factor).

Justme
vegplot

Justme wrote:
JB wrote:

(BTW Mrs JB has just asked what I was talking about as she could hear me typing away. When I told her she just said "I'm so glad I asked" Very Happy )


I get that A LOT
Lol

vegplot wrote:

Where are you getting the 53VA figure from?


I am guessing he has a power meter that plugs into the socket like mine that shows volts, amps, watts, kwh, hours, VA, PF (power factor).

Justme


In which case he is using a laptop with an inefficient power supply or a faultly power meter Smile
RichardW

vegplot wrote:


In which case he is using a laptop with an inefficient power supply or a faultly power meter Smile




My fridge is using 1 watt whilst not chilling but the VA is 10

entertainment system 17watt 34 VA on standby &
150watts 174 VA when on

My chip fryer has almost identical reading around 2165watts / VA

Justme
vegplot

Justme wrote:
vegplot wrote:


In which case he is using a laptop with an inefficient power supply or a faultly power meter Smile




My fridge is using 1 watt whilst not chilling but the VA is 10

entertainment system 17watt 34 VA on standby &
150watts 174 VA when on

My chip fryer has almost identical reading around 2165watts / VA

Justme


The chip fryer is almost a purely resistive load and will have a power factor close to 1. However, the fridge is inductive and have a lower power ractor, an entertainment system will be a mix of resistive, inductive and capacitance loads.

"Power factor (PF) is the timing relationship between voltage waveform and current waveform. If they are in sync the PF 1.00. Generally, only heaters and many common light bulbs have a perfect PF of 1.00. For a refridgerator, a PF of 0.30 (30) is not very good. It will be due to the inductive compressor motor. A poor power factor (less than 1) can mean that you pay for more power than you actually use because the the current draw is higher to achieve the same apparent power. Things like computers and monitors (switchmode power supplies) cause a distorted current waveform which complicates the power reading."
RichardW

The plot thickens.

Its not a simple as doing the numbers then.

Justme
vegplot

Justme wrote:
The plot thickens.

Its not a simple as doing the numbers then.

Justme


Not quite Sad

But don't get sidetracked by it. If you have a power meter use the wattage reading. Use VA if you want to know the cuurent flowing through the system. Depends on what you want to use the readings for.
RichardW

How am I billed?

I thought it was kwh / units which suggest that watts time hours used, but if its calculated by using the VA then we are getting billed for more due to the way its calculated.

I seem to remember a whole house gizmo that will reduce the VA / improve the PF so small meters like the ones we are using will show less usage but the supply meter still will read the same amount used / billed as it is not that cleaver.

If I want to size an of grid solution which is the best number to use?

Justme
vegplot

Justme wrote:
How am I billed?

I thought it was kwh / units which suggest that watts time hours used, but if its calculated by using the VA then we are getting billed for more due to the way its calculated.

I seem to remember a whole house gizmo that will reduce the VA / improve the PF so small meters like the ones we are using will show less usage but the supply meter still will read the same amount used / billed as it is not that cleaver.

If I want to size an of grid solution which is the best number to use?

Justme


Now you've got me. I'm not sure but it is likely to be VA in which case that's what you want to measure.

For an off grid solution I would advise going with the higher value if you have a lot of inductance loads such as chilllers. If you're primary loads are resistive then it's safe to use the measured power.
JB

Justme wrote:
vegplot wrote:

Where are you getting the 53VA figure from?


I am guessing he has a power meter that plugs into the socket like mine that shows volts, amps, watts, kwh, hours, VA, PF (power factor).

Justme


Indeed I have.

vegplot wrote:
In which case he is using a laptop with an inefficient power supply or a faultly power meter Smile


Is it that inefficient? As the power supply is a transformer (ie a stonking great inductor) it will introduce a phase difference. That gives a figure of about 0.65 for the powerfactor which is about the same as when I try the same exercise with a desktop PC. I also get about the same power factor with everything shutdown so that the only power drains should be the WAP, router and any quiescent drain from the transformers for the monitor, laptop and DAB radio even if they are shut off. Obviously the power figures themselves vary but the PF appears to be always about 0.65. (I have tried it on a purely resistive load and it does give PFs of about 1)

vegplot wrote:
But don't get sidetracked by it. If you have a power meter use the wattage reading. Use VA if you want to know the cuurent flowing through the system. Depends on what you want to use the readings for.


Two things I want to work out - how much money I would save and the environmental impact. From what I have read it would appear that we are charged in watts but the environmental impact would arise from VA. This is based on the fact that the generators seem to get upset if you use large amounts of energy with a poor PF.
vegplot

JB wrote:
Justme wrote:
vegplot wrote:

Where are you getting the 53VA figure from?


I am guessing he has a power meter that plugs into the socket like mine that shows volts, amps, watts, kwh, hours, VA, PF (power factor).

Justme


Indeed I am.

vegplot wrote:
In which case he is using a laptop with an inefficient power supply or a faultly power meter Smile


Is it that inefficient? As the power supply is a transformer (ie a stonking great inductor) it will introduce a phase difference. That gives a figure of about 0.65 for the powerfactor which is about the same as when I try the same exercise with a desktop PC. I also get about the same power factor with everything shutdown so that the only power drains should be the WAP, router and any quiescent drain from the transformers for the monitor, laptop and DAB radio even if they are shut off. Obviously the power figures themselves vary but the PF appears to be always about 0.65. (I have tried it on a purely resistive load and it does give PFs of about 1)

vegplot wrote:
But don't get sidetracked by it. If you have a power meter use the wattage reading. Use VA if you want to know the cuurent flowing through the system. Depends on what you want to use the readings for.


Two things I want to work out - how much money I would save and the environmental impact. From what I have read it would appear that we are charged in watts but the environmental impact would arise from VA. This is based on the fact that the generators seem to get upset if you use large amounts of energy with a poor PF.


A laptop/desktop is only a small part of your total energy needs and therefore you do not need to be concerned too much about PF. The lower the PF the less efficient a device is - in general. Most loads are resistive.

Your question "how much money I would save" save by doing what? I haven't grasped that.

This is quite a good page on the topic, you may have read it already.

http://www.adsm.com/energyfactor.php
JB

vegplot wrote:
Your question "how much money I would save" save by doing what? I haven't grasped that.

This is quite a good page on the topic, you may have read it already.

http://www.adsm.com/energyfactor.php


Ta for the link.

There's two points I'm looking at one is to pitch energy savings to companies in terms of the cost saving to themselves. The other is to pitch the same energy saving in terms of the environmental impact.

Yes computers and similar may be a small part of the total energy that a company will consume but they are also an area where people forget to make those savings. Because of the poor PF in that sort of equipment I'm wondering if the environmental impact is greater than a simple measurement of the cost would suggest.
vegplot

JB wrote:
There's two points I'm looking at one is to pitch energy savings to companies in terms of the cost saving to themselves. The other is to pitch the same energy saving in terms of the environmental impact.


I think I know where you're coming from. In which case use the VA calculation.
JB

vegplot wrote:
JB wrote:
There's two points I'm looking at one is to pitch energy savings to companies in terms of the cost saving to themselves. The other is to pitch the same energy saving in terms of the environmental impact.


I think I know where you're coming from. In which case use the VA calculation.


Because it's more accurate or because it's bigger? Very Happy
vegplot

JB wrote:
vegplot wrote:
JB wrote:
There's two points I'm looking at one is to pitch energy savings to companies in terms of the cost saving to themselves. The other is to pitch the same energy saving in terms of the environmental impact.


I think I know where you're coming from. In which case use the VA calculation.


Because it's more accurate or because it's bigger? Very Happy


Both Smile
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