Take the 2-minute tour ×
Super User is a question and answer site for computer enthusiasts and power users. It's 100% free, no registration required.

Does my 1000 watt 80gold+ (90% efficient) PSU pull 1111 watts, 1000 watts, or 900 watts from my wall receptacle at 100% load?

When I plug my 200 watt video card into one of the PCIe power connectors... is it pulling 222, 200 or 180 watts from the PSU?

Why does my PSU claim 1000 watts continuous and 1100 watts max. Which is it? How many watts can I pull out of this thing?

share|improve this question
2  
A link to the specific model of PSU would be extremely useful here. The spec sheet on the PSU would almost certainly reveal a lot more detail then just saying 1000 watts. –  Zoredache Jun 5 '11 at 20:39
2  
It just means you can pull 1000 watts at all times, but if you spike to 1100 it wont blow up. –  Nate Jun 5 '11 at 20:53
    
I do not have a specific model of PSU in mind - and to keep the question timeless I would rather not provide one. –  darkAsPitch Jun 6 '11 at 3:27
    
+1 to Nate - Keep it simple! –  Linker3000 Jun 6 '11 at 9:52
2  
If the PSU is supplying 1000 W, and you know the efficiency at 100% load is 90%, then it is pulling ~1111 W from the wall. The extra 111 W are dissipated due to resistive component losses (i.e. heat). –  Breakthrough Jun 28 '11 at 14:14
add comment

4 Answers

up vote 3 down vote accepted

The efficiency is an extremely complex topic, and in very simple terms Wil's answer is correct, but...

The test specifies following parameters:

87% efficiency at 20% of stated load, 90% efficiency at 50% of stated load, 87% efficiency at 100% of stated load and power factor of 0.9 at 50% of load. Voltage level is expected to be 115 V.

How the power factor part goes into this isn't immediately clear.

So what does all of that mean?

Well, let's take a look at test protocol.

From it we learn that the efficiency is considered ratio between real power provided at device output and real power provided at device input. This is important when calculating whole power consumption of the device. From it we can see that the power factor isn't taken into concern when the efficiency is calculated. So at 50% load, we can more or less accurately calculate total power consumption of the device: First, we say it provides 500 W. Then we take efficiency in, so we know that it consumes around 555.56 W of power. At last, we take power factor into equation and we get that it consumes around 617.28 VA. That is the total power consumed by the device. The real power power supply consumes is 555.56 W and it also consumes 61.72 VAr for which you may or may not be charged, depending on the way electricity is sold in your area.

We can't be sure what the total power consumption will be at 100% load, be cause we can't take into effect the power factor which is unknown at the 100% load. I don't think that it will be considerably lower than the on at 50% and it may be higher.

Also voltage level can make an impact at efficiency of the power supply. Power supplies running at 230 V are usually a bit more efficient than when running at 115 V, but the results will vary from PSU to PSU.

Also about the card power consumption. Here, card is considered a separate system and its own efficiency isn't taken into account. Instead, you're provided with its maximum power consumption. So a 200 W card will pull up to 200 W from the power supply. However, if the power supply is 90% efficient at the moment, you could say that the card will pull 220 W form the power plug of the computer plus an unknown number of volt-ampere reactive.

share|improve this answer
    
Most people do not get billed for VAr consumption. Real power does real work, which costs real dollars. The same can be said for imaginary power, which does imaginary work, which costs you imaginary dollars. ;) –  cp2141 Jun 30 '11 at 13:50
    
@cp2141 Well I get billed real dinars for very real reactive power. Furthermore the "imaginary work" statement is incorrect. Reactive power causes problems with power transmission systems which require real work to be done in order for them to be mitigated. While some people may not be charged for reactive power, it is a real concern and it is quite unfortunate that many people don't realize it. –  AndrejaKo Jun 30 '11 at 14:31
    
I never said it was not a real concern (I work in power systems, I know the problems reactive power causes). Where I live, we don't get billed for reactive power, so I did not know that in some parts of the world you are billed for your power factor. That being said, the work done to fix the power system does not require real work (we use capacitor banks or alter the field winding current to mitigate reactive power effects). –  cp2141 Jul 2 '11 at 23:36
add comment

If it quotes it is 1000 watt with a 1100 watt max, this typically means that it will support 1100 watts all the time but it is not recommended at all and will shorten the life of the unit. It is only rated at 1000 watts and you should never go above that (as much as it supports it)... and unless you have many high end graphics cards, drives and more, you are unlikely to go above 1KW.

When you plug in a 200 watt video card, it will most likely pull about 40-50 watt idle and go up to a maximum of 200 watt if used at 100% load (if the specifications say it has a maximum of 200watt).

For the graphics card and the system in general, at 100% load (not the maximum 1100, but the supported max of 1000 watt), will most likely pull about 1,100 watts - meaning the system is using 1,000 watts along with a further 10% loss as heat.

A further example is a 1,000 watt machine with 80% efficient means it is 20% inefficient and will pull 1,200 watts.

(So to go back a bit, a 200watt card will actually pull 220 watts at 90% efficiency)

Again, remember, these are the maximums, machines fluctuate greatly whilst they are on - accessing an optical drive, being idle, moving the mouse (all be it minimal)... anything will change the wattage of the machine

share|improve this answer
    
@Paradroid pointed out in chat that my calculations for electricity usage is wrong - after reading Wikipedia, I think it may be as well... I thought I understood but I may be incorrect - Please feel free to edit my answer or add your own if you can do better :/ ... trying to find another write up to clarify at the moment, so please take this answer as maybe for now... –  William Hilsum Jun 5 '11 at 21:18
add comment

The answer form @AndrejaKo is very good and detailed but I feel it is a little complex so I'll try to answer the question as simple as possible:

The important things to know:

  • The 80gold+ rating guaranties 90%+ efficiency between 20% and 80% of its capacity.
  • The efficiency loss is happening in the transformation of the AC power from the wall to DC power that drives the equipment.
  • A 1000w rating is the addition of the multiple voltage lines on the PSU. For instance it may have one 3.3v line + one 5v line + three 12v lines that are each rated at 200w for a total of 1000w. So it is difficult to draw the exact 1000w of the PSU because each accessories may use a different line. But in general you don't have to take care of this when calculating PSU requirements because you will usually try to match the maximum power usage to 80%-90% of the capacity of the PSU, hence you have some overhead.

Now to answer the exact questions:

A 1000w PSU rated at 90% efficiency at full load will draw 1111w of 120v AC from the wall. This 1111w AC will be converted at 90% to 1000w DC and the rest will be lost in heat. But remember that it is most likely that efficiency at full load is much lower than 90%.

If you plug a 200w card to the PSU, it will draw 200w DC from the PSU because there is no loss at this stage as it was converted from AC in the previous calculation.

The 1000 continuous and 1100 max means that it is capable of providing 1000w constantly but will accept spikes up to 1100w. Such spikes may occur for instance when an optical drive starts to turn or some new devices are connected to USB.

share|improve this answer
    
There are losses when transmitting any kind of power, AC or DC. DC power is known to have even higher line losses then DC. While your 200W graphics card may won't draw any more then 200W, it will be provided with slightly less due to the (negligible) losses in the PCI-E power wires. Yes, it is negligible, but it's an important fact nonetheless. –  cp2141 Jun 30 '11 at 13:53
add comment

Does my 1000 watt 80gold+ (90% efficient) PSU pull 1111 watts, 1000 watts, or 900 watts from my wall receptacle at 100% load?

If the efficiency is 90% at full load (1000 W), the power supply will draw 1111 W from your wall socket. When the power supply states delievered power, they mean the power delievered to the computer components, because the expectation is that your wall socket is more then capable of delivering the required power.

That being said, most 80 Gold hit their peak efficiency at about 50% load. The unit begins to saturate near full load, and the efficiency typically drops (see this link, click the "Tech Specs" for an example). Furthermore, the efficiency is typically a function of the input supply voltage.

When I plug my 200 watt video card into one of the PCIe power connectors... is it pulling 222, 200 or 180 watts from the PSU?

It is pulling 200 W. Whenever we deal with net power delivered from the power supply's perspective, we are talking about how much power the connected components draw. Computer components don't draw enough current to have any substantial losses in the wires.

Why does my PSU claim 1000 watts continuous and 1100 watts max. Which is it? How many watts can I pull out of this thing?

1100 W is the peak load you can place on the power supply before the unit begins to saturate. Computer power supplies are an example of a switched-mode power supply. They basically output a rectified high-frequency waveform, and vary the supplied power by varying both the output frequency and duty cycle.

When the manufacturer specifies 1100 W as the maximum load, that is the maximum possible power the unit can supply before saturation. Any attempts to draw more then that will simply result in excess losses through heat, and unadequate power supply to your computer's components (the voltage levels may rise/fall, damaging your components). Depending on the design of the unit, this may trigger one of the PSU's failure modes, causing it to shut off or blow an internal fuse.

share|improve this answer
add comment

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.