Power Consumption

[zahinhasan]

I have a 700W power supply. Does it take 700W from the wall all the time and deliver whatever necessary according to the task or takes whatever necessary from the wall?(Just want to know the concept so I didn't factor the efficiency)  

Also if I use the hardware monitor to check the power usage what will I get? what the PC is drawing from the wall or what the PC is using or are they same? Can this value be used for the calculation for the electricity cost? 

[Chris Pratt]

No. It draws the power the components need at the moment, and that's all.

[SkilledRebuilds]

Starters.. plenty more info to be given but here is a quick and dirty.
PC will only ever use what it uses.. a 700w unit is not pulling 700w live,..
Using software to monitor particular parts and seeing.. CPU power or GPU power...
CPU Power, is for CPU only.
GPU Power, is for GPU only.

You'll get more examples as replies shortly I'm sure,..
 

[Lurick]

No, a PSU draws only what is needed to supply power required by the components. It does not draw full wattage. If you want to know how much it's pulling from the wall at any given time a Kill-A-Watt or other power meter is what you'll want to look into.

[Donut417]

8 minutes ago, zahinhasan said:

Does it take 700W from the wall all the time and deliver whatever necessary according to the task or takes whatever necessary from the wall?

The 700 Watt rating is just the max peak wattage the PSU can supply. Your activities on your computer will determine what each component needs. So effective the computer only draws the amount of power it needs at any given time. Further more PSU's converts AC to DC and the conversion is not 100% efficient. Which means a 700 Watt PSU can draw more than 700 Watts from the wall. This is the reason the 80+ standard exists. 

[zahinhasan]

5 minutes ago, Lurick said:

No, a PSU draws only what is needed to supply power required by the components. It does not draw full wattage. If you want to know how much it's pulling from the wall at any given time a Kill-A-Watt or other power meter is what you'll want to look into.

I don't have Kill-A-Watt or that kind of meter near my area. So any software solution?

[Donut417]

2 minutes ago, zahinhasan said:

So any software solution?

Software wont likely work. Because all it can do is see the draw your components have. The problem is the PSU draws more from the wall than what the components need. This is because the PSU is not 100% efficient in converting AC to DC. So if you want 100% accurate results it needs to be an external box. Such as Kill-A-Watt meter or a UPS that can read power draw. 

[manikyath]

first: the rating of a power supply is the maximum it can provide on it's output.

so if your pc needs 300 watts, it'll take 300 watts, if your pc needs 700 watts it'll take 700 watts, if your pc needs 900 watts, the power supply's protection circuitry will kick in and shut things down.

 

past that.. the power it actually pulls from the wall, is based on the power pulled by pc components, and the PSU's efficiency curve. 80+ ratings give you a "minimum efficiency" at certain loads. for example 80+ gold requires 87% efficiency at 20% load, 90% efficiency at 80% load, and 87% efficiency at 100% load.

in turn.. for a 700 watt 80+ gold power supply:

- at a 140 watt load (20% of rated power) it'll pull at most 160 watts from the wall

- at 560 watt load (80% of rated power) it'll pull at most 622 watts from the wall

- at 700 watt load (100% of rated power) it'll pull at most 804 watts from the wall.

 

however, good to note is that that's efficiency calculated between the PSU's input side (mains plug) and output side (the connectors coming out the power supply)

this doesnt factor in motherboard VRM efficiency, losses in extension cables, etc.

 

as for the second part of your question: what do those sensors actually measure?

well.. it depends... it could be a "measured" value, or a calculated value based on other factors, or just software based wild guesses based on the chip's current power state (voltage, multiplier, etc.)

it could be that your motherboard VRMs measure the power they're taking in, it could be they're measuring the power they're feeding to the cpu..

 

all in all.. those values arent really a useful "value", past comparing them to themselves in different scenarios.

 

as for the answer to calculating electricity cost.. the solution is annoying but painfully simple:

there is no good software way to decide power draw from the wall, except for those few premium power supplies that actually have builtin power meters.

Go to your local home improvement store, and ask for an "energy cost meter", they only cost maybe 30 bucks at most, and allow you to program your power grid tariffs right into them, providing you with actual "cost per hour", and "total cost" values that are pretty darn accurate.

[zahinhasan]

Just now, Donut417 said:

Software wont likely work. Because all it can do is see the draw your components have. The problem is the PSU draws more from the wall than what the components need. This is because the PSU is not 100% efficient in converting AC to DC. So if you want 100% accurate results it needs to be an external box. Such as Kill-A-Watt meter or a UPS that can read power draw. 

Thanks for the answer.

[mariushm]

 

The power supply takes some amount of power  from the AC socket in your house and converts it to DC voltage and then produces the voltages the computer wants.. modern computers use 3.3v, 5v and 12v.

This conversion is not 100% efficient, depending on how much power components demand, the efficiency is around 80-95%.

 

The power supply only produces as much DC power as the components demand, so for example, if your components in total need 200 watts, your power supply will take around 220 watts from the AC socket and produce the power demanded.

 

You can use some tools like HWInfo to approximate how much power components consume,  IF those components report the power they consume (volts x current = power in watts).  SO for example, if you see the video card saying it consumes 5A, then you know the video card is powered from 12v, therefore you can estimate the video card consumes 12v x 5A = 60 watts.

Keep in mind that complex components like a video card will have multiple power supplies, one or two for the actual processor and one for the memory, so you may see several amounts of power reported by the video card.

 

You can also look on labels of various products to estimate how much they produce ... for example if you have a SATA SSD that says  power consumption : 5v 2A , you know the most it's gonna consume is 10 watts.

For other components, you may be able to look reviews ... for example for a video card, doesn't have to be the exact video card model, your video card will consume about the same amount of power.

 

 

[zahinhasan]

Just now, manikyath said:

first: the rating of a power supply is the maximum it can provide on it's output.

so if your pc needs 300 watts, it'll take 300 watts, if your pc needs 700 watts it'll take 700 watts, if your pc needs 900 watts, the power supply's protection circuitry will kick in and shut things down.

 

past that.. the power it actually pulls from the wall, is based on the power pulled by pc components, and the PSU's efficiency curve. 80+ ratings give you a "minimum efficiency" at certain loads. for example 80+ gold requires 87% efficiency at 20% load, 90% efficiency at 80% load, and 87% efficiency at 100% load.

in turn.. for a 700 watt 80+ gold power supply:

- at a 140 watt load (20% of rated power) it'll pull at most 160 watts from the wall

- at 560 watt load (80% of rated power) it'll pull at most 622 watts from the wall

- at 700 watt load (100% of rated power) it'll pull at most 804 watts from the wall.

 

however, good to note is that that's efficiency calculated between the PSU's input side (mains plug) and output side (the connectors coming out the power supply)

this doesnt factor in motherboard VRM efficiency, losses in extension cables, etc.

 

as for the second part of your question: what do those sensors actually measure?

well.. it depends... it could be a "measured" value, or a calculated value based on other factors, or just software based wild guesses based on the chip's current power state (voltage, multiplier, etc.)

it could be that your motherboard VRMs measure the power they're taking in, it could be they're measuring the power they're feeding to the cpu..

 

all in all.. those values arent really a useful "value", past comparing them to themselves in different scenarios.

 

as for the answer to calculating electricity cost.. the solution is annoying but painfully simple:

there is no good software way to decide power draw from the wall, except for those few premium power supplies that actually have builtin power meters.

Go to your local home improvement store, and ask for an "energy cost meter", they only cost maybe 30 bucks at most, and allow you to program your power grid tariffs right into them, providing you with actual "cost per hour", and "total cost" values that are pretty darn accurate.

Thanks for the detailed explanation.

[Chris Pratt]

1 hour ago, Donut417 said:

Software wont likely work. Because all it can do is see the draw your components have. The problem is the PSU draws more from the wall than what the components need. This is because the PSU is not 100% efficient in converting AC to DC. So if you want 100% accurate results it needs to be an external box. Such as Kill-A-Watt meter or a UPS that can read power draw. 

Also software is only as good as the sensors and motherboard in general are at reporting correct info. In some cases, the mobo will intentionally report less power utilization than is accurate to try to juice the connected components into performing better. The only way to know for sure is to measure at the 12V cable or at the wall using appropriate electrical equipment.

[Donut417]

3 minutes ago, Chris Pratt said:

wall using appropriate electrical equipment.

Which is what I said. 

 

1 hour ago, Donut417 said:

So if you want 100% accurate results it needs to be an external box. Such as Kill-A-Watt meter or a UPS that can read power draw. 

 

[Chris Pratt]

Just now, Donut417 said:

Which is what I said. 

 

 

I realize that. I was reiterating your point, by saying the can't trust the software readings.

[mariushm]

You may also not get the conversion efficiency losses in the dc-dc converters and you won't get the losses in the actual wires between the power supply, and the losses caused by contact resistance (in the connectors)

 

The simplest would be to measure the power pulled from the mains socket using a kill-a-watt type device. It will give you a reasonable result.

 

If you want actual component consumption, best would be to measure the current with an actual multimeter placed in series with the power input (the eps connector, the pci-e 8pin connectors).  If that's too difficult, a clampmeter with good DC current measurement would also work fairly accurately.

 

But... video cards are a bit tough because they get power from both slot and separate power connectors... so you'd have to use a pci-e riser cable or something that allows you to pump 12v and 3.3v through separate inputs.