# PSU question

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November 3, 2009 3:38:23 AM

Will a 400 watt psu spend more energy than a 300 watt psu for the same hardware setup?

I assume the 400 watt psu will spend more energy, at least in low demand state...

I saw a thread of a guy saying he had a 300 watt generic psu that draw 85 watts with his hardware, and the same hardware with an antec 80+ draw 79 watts, and he ended his comment by saying that was like 75 effiency...

a) can someone explain this a little more, which one is more efficient and how did the figure the 75?
b) how was this guy able to measure the wattage draw in both cases?

Thanks

a b ) Power supply
November 3, 2009 3:51:30 AM

You measure the wattage with a watt meter plugged into the outlet.

The efficiency of a PSU can have an effect on how much it draws. Poor low end efficiency will draw more power than a PSU will good low end efficiency.

300 watts or 600 watts. If the system only needs 200 watts to operate, the PSU with the higher efficiency will draw less power to run the system.
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a b ) Power supply
November 3, 2009 4:02:38 PM

Suppose PC 85W for its operation.

A good 500W PSU with efficiency of 85% will draw 100W from wall outlet and give 85W to the PC.

Also a good 400W PSU with efficiency of 85% will draw 100W from wall outlet and give 85W to the PC.

But a poor 350W generic PSU with efficiency 50% will take 170W from the wall outlet and furnish 85 Watts.

A 500W PSU won't take any more power than a 400W unit with same efficiency.
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November 3, 2009 7:03:16 PM

khmer said:
Will a 400 watt psu spend more energy than a 300 watt psu for the same hardware setup?

I assume the 400 watt psu will spend more energy, at least in low demand state...

This all depends on the hardware setup, and how much power it draws.

khmer said:
I saw a thread of a guy saying he had a 300 watt generic psu that draw 85 watts with his hardware, and the same hardware with an antec 80+ draw 79 watts, and he ended his comment by saying that was like 75 effiency...

a) can someone explain this a little more, which one is more efficient and how did the figure the 75?
b) how was this guy able to measure the wattage draw in both cases?

Thanks

I will explain more in another post below. I haven't seen the thread you are referring to, but it isn't possible to give a single efficiency figure, as I'll explain below.

The question about measure the power consumption, plug a power meter in between the wall and your PSU.
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a c 163 ) Power supply
November 3, 2009 7:31:42 PM

PSU's are generally more efficient at 3/4 load than at full load. So at 300w draw, all things being equal, a 400watter would be more efficient than a 300 watter.
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November 3, 2009 7:55:38 PM

It is very important to understand that the efficiency of a power supply is not flat, but a dome shaped curve. Efficiency varies depending on the percentage of max power the PSU is working at.

The efficiency figures often quoted by manufacturers are the top of the curve - its maximum effiency. This often occurs at approxiamately 50-55% of the max rated load of the PSU. For a 500W PSU this would be around 250W load, and the effiency at this load would be around perhaps 85% for a fairly good one.

At 480W load (top end), and at 20W load (bottom end) the 500W PSU will perform at worse than 85% effiency. You will notice, if you look into the different 80 Plus gradings (ordinary, Bronze, Silver and Gold) that they specify different efficiency hurdles at different percentages of max load, so:

edit: This table hasn't come out neatly, because the forum is auto-deleting multiple spaces. I don't know the syntax to stop that.

20% 50% 100% of Max PSU rating

Ordinary 80 Plus 80% 80% 80%
Bronze 82% 85% 82%
Silver 85% 88% 85%
Gold 87% 90% 87%

You'll notice:
1) The drop-off in efficiency that 80 Plus expect at the top end is quite subtle
2) They don't specify any required efficiency level when the PSU is operating at less than 20% of max load. That is because the drop-off at the bottom end is usually quite sharp. 80 Plus accept that the performance of most PSUs is poor at much less than 20% of their capacity.

Point 2) above is not well understood, and people can make a mistake here. Wanting scope for extra GPUs, overclocking etc they go and buy a very high capacity PSU 'to give lots of headroom'. Worried about effiency, they find one with an 80 Plus Silver rating, perhaps a 1000W power supply.

The power consumption of a computer varies depending on whether it is nearly idle browsing the web, or worked hard to play the latest game.

At idle the computer might well only be drawing about 130W or less (even a midrange system if its upto date). Don't believe me? Check out the ATI Radeon 5750 system on this graph from a Tom's Hardware review:

Now at just over 100W drawn from a 1000W PSU, that's a 10% load - not a level at which any PSU works optimally. Even for our 80 Plus Silver rated PSU, the effiency level may have dropped off to 70% or less at this point.

Now at maximum load (look back at the graphs on the review I posted) the midrange system might be up to about 200W or a bit less. If its silver rated then we've got up to 20% load so we should now be hitting 85% efficiency. BUT THAT IS MAX COMPUTER DRAW. How much of the computers life is really going to be spent at that kind of workload? Normally not much. So therefore in this example the computer is mostly going to be drawing at less than 20% of what the PSU can provide - and this is any PSUs worst performing area.

The best thing to do is to choose a power rating for your PSU so that it operates in its sweet spot around 50%. Look at the power consumption graphs within TomsHardware reviews of graphics cards like yours, with CPUs like yours, if possible. Make adjustments down where you know your CPU is less power hungry.

Take your guesstimated power draw at idle (say 150W) and your guesstimated power drawer when your computer's working hard (say 220W).

Now, if you get a 400W power supply, your computer will be drawing at 37.5% of PSU capability at idle and 55% of PSU capacity when working hard. The PSU would then be operating in its effiency sweetspot. So in this example you would restrict your search to PSUs of 400-500W rating, THEN WITHIN THAT you look for one with the best 80 Plus rating.

An appropriately powered PSU with a bronze rating will likely outperform a silver rated PSU with an excessive power capability, because in that case the silver is operating well below its sweetspot.

Where a PSU doesn't have an 80 Plus rating, but just quotes its max effiency (remember usually this happens at 50% of rated load), you may find that the efficiency curve is more 'domed', with a worse drop-off at each end. In this case it is even more important to get the right power.

Of course, err slightly on the high side with power rating, and do make allowances for up-speccing your machine, but only if you are serious about doing that in the near future.

That's my exhaustive guide to maxing your PSUs effiency with your particular computer. Its a bit long, I may edit it down later.
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November 3, 2009 8:57:48 PM

khmer said:
Will a 400 watt psu spend more energy than a 300 watt psu for the same hardware setup?

I assume the 400 watt psu will spend more energy, at least in low demand state...

So to answer your exact question, yes you are right if the hardware setup has lowish power requirements of say 70W idle. 70W is 23% of 300W (OK), but 17% of 400W (sub-optimal). Your point would have had more force if you'd referred to 300 and 600 rather than 300 and 400 which are fairly close.

khmer said:
I saw a thread of a guy saying he had a 300 watt generic psu that draw 85 watts with his hardware, and the same hardware with an antec 80+ draw 79 watts, and he ended his comment by saying that was like 75 effiency...

a) can someone explain this a little more, which one is more efficient and how did the figure the 75?

If you change the PSU and the resulting power draw is less, then you know it must be more efficient. To know the exact efficiency a given power supply is operating at with particular hardware you have to know

1) What power the hardware is drawing from the PSU
2) What power the PSU is drawing from the wall

To know 1) is quite difficult, 2) is much easier (with a device that plugs in between the wall and the PSU).

In your example, we might imagine the hardware is drawing 70W. Due to ineffiency the PSU will have to draw more than this from the wall to allow for its wasted energy (mostly heat). The less the inefficiency, the less it has to drawer from the wall, and the closer to 70W that will be. If it only had to draw 70W from the wall to provide the computer with same 70W then that would be 100% efficiency.

Not enough information has been provided to prove/disprove the 75% efficiency claim.
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a c 144 ) Power supply
November 5, 2009 11:28:21 AM

b82 said:

To know 1) is quite difficult, 2) is much easier (with a device that plugs in between the wall and the PSU).

No. 1 is not exactly difficult. It just requires test equipment (a clamp-on ammeter) that is not commonly available.

For instance, I know that my Q6600 pulls 8 amps at stock speed and 9.5 amps when OC'd to 3.6 GHz through the CPU power plug. I took those measurements by pushing back the sleeveing and clamping the ammeter around the yellow wires.

Repeat for all the different cables and colors and total the results. For convenience, you need to do this on a breadboarded system. And it's tedious, but not particularly difficult.

Hey. I was curious one day.

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November 11, 2009 3:35:32 PM

jsc said:
I took those measurements by pushing back the sleeveing and clamping the ammeter around the yellow wires.

Repeat for all the different cables and colors and total the results. For convenience, you need to do this on a breadboarded system. And it's tedious, but not particularly difficult.

OK, you're right it is not VERY difficult, but it is certainly more difficult than the simple power meter that you can plug in between the PSU and the wall socket. Also you have not correctly described how to get the power figure I mentioned in point 1) above.

To get an efficiency figure you need to compare POWER taken from the wall with POWER taken from the PSU inside the computer. You've described how to measure the CURRENT (measured in amps) drawn inside the computer.

As I understand it, you should not just add up those current measurements that you described. The different components draw at different voltages. Power = current x voltage so you should multiply each current reading in amps by the voltage that is supplied to that same component.

Only when you have done these multiplications can you finally add up all the component power figures to get total power drawn from the PSU by your system.

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a c 144 ) Power supply
November 13, 2009 11:41:33 AM

Yes. I thought it was obvious that after you have the current demands on the various outputs that it was then a simple exercise to total the watts - not the current, then divide that by the input power reading to calculate the efficiency at that power point.

I just assumed that someone who is discussing efficiency at this level knows the difference between input and output and the difference between volts, amps, and watts.

Maybe I was wrong.
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