Power Draw questions - Overkill vs. Minimums

[Hallis]

I've been considering a new power supply for my main rig and retiring my old Enermax 650 to server duty. I've read a lot of good information in here about requirements and whatnot.But since ive read that PSU's operate best and live longest at between 40%-60% usage I was considering going with a fairly large capacity unit like a 1kw.

Now that being said i'd like to find out about a few more comparisons. I dont need specific scientific data but id like to know..

1. If i upgrade to a 1kw versus my existing 650w, Should my PC still draw roughly the same ammount of power from the wall?

2. Because in theory it would be putting less of a load % on the PSU would the 1kw generate less heat than running the 650w at closer to capacity?

I realize it's sortof an Apples to Oranges comparison since I dont have specific model #'s to compare but Would probably go with an Enermax or Corsair PSU as they both seem to be fairly well built and reliable.

The current system with the 650w (which im looking to upgrade the PSU in) is a Core i7 at stock speeds (dont plan on overclocking but I may slightly at some point), 4870x2, WD Velociraptor 300gb, 1xDVDRW, And 5x 120mm fans.

Any advice would be helpful.

Shane

 

[BillParrish]

1. If i upgrade to a 1kw versus my existing 650w, Should my PC still draw roughly the same amount of power from the wall?

Yes, if the efficiency of the units (turning AC into DC) is about the same at the same delivered power levels the draw from the will be the same.

Say your machine actually needs 400W of DC power to do "something" if one power supply was 80% efficient providing 400W DC the draw from the wall would be 400 / .80 = 500W AC , if the other power supply was say 83% efficient providing 400WDC the draw from the wall would be 400 / .83 = 482 W AC a little less because the second supply was a little more efficient in this example but the draw from the wall is roughly the same. If the two units where both 80% efficient then the draw from the wall would be exactly the same. (Note to EE's yes, a little simplified as we do not take the phase angle into consideration, lets just assume both units have perfect power correction. )

2. Because in theory it would be putting less of a load % on the PSU would the 1kw generate less heat than running the 650w at closer to capacity?

No, if both units have about the same efficiency and you are generating say using our number from above 400W of DC from AC for both units, while pulling more than that in AC from the wall. The difference in DC generated vs AC consumed is the efficiency of the unit and looking at it from another way, if a unit is 80% efficient there must be 20% loss. So if the big supply and the little supply where both 80% efficient both supplies would have 20% loss in the conversion process of converting AC to DC and puting out our made up 400W, that 20% loss of power is converted to heat in the process of doing work (the power supply does not move or emit significant radiation - the other ways energy is converted to work off the top of my head- so the losses are heat). Hope you still with me. So the loss of both supplies is about the same, the work done by both units is 400W and the same, and the loss gets converted into heat so the amount of heat will be about the same. ( In theory exactly the same)

Second shot at explaining, hopefully simpler.
Both units have same effeciency or close, both putting out 400W DC or whatever same power, so both pulling about same from wall and losses the same for both units , loss is heat, loss the same so heat generated the same.
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The advantage is the bigger rated supply was designed to do more work (640W vs 1000W) and so its components can be expected to be "beefier" and so while the heat generated in the conversion process is the same the larger supply is in theory better able to handle it with less stress.

Peanut Galley.
I would do a price/watts on units between 800W and 1KW. There should be a point where the cost per watt jumps. What I am trying to say is that an 850W unit might both give you the "overhead" you are looking for and be significantly cheaper. But then again someone might have a 1KW unit on sale. I over engineer stuff all the time but there is a point where it becomes silly and a waste of money if there are not plans in the future to use the capability. For example the big caps in the 1KW supply might be rated 105C and better than caps in an 850W supply rated for 80C but if the 1KW supply fails because of some other cause the big bucks spent on those 105C caps and what you thought was piece of mind do not matter. Gah enough. you get my drift. I recommend over specing, just don't go crazy and look for a deal without compromising quality.

 

[Hallis]

Very good info there Bill.

One more question on that. Since efficiency effects heat generated, Are PSU's more efficient at lower loads? For instance arent the 80%, 82%, etc. ratings based on full load if im correct. So wouldnt a PSU be more efficient say at 50% load?

 

[BillParrish]

Excellent question and something I forgot to mention. Usually yes but it depends on the design. You are spot on, I forget how much load is on the PSU when they state the efficiency for the ratings. But you want to be looking for more detailed info, a chart of load vs efficiency or a "efficiency at %50 load" number. I think some of the reviews do that.

Anyway, you most certainly need to look at that, the efficiency at the load level you expect the new power supply to actually be running/supplying DC at.

I cant give a definitive answer as I have not been keeping up with the newer supply's and designs, it was a slow day over in OCing and I was bored. Someone be along that knows off top of head.

 

[Hallis]

I'll check over some of the recent reviews and see what i can come up with

thanks again for the good info Bill. I'm well read on a lot of PC hardware but power supplies are one area where I've not even scratched the surface.

 

[Tsumi]

Efficiency tends to run in a curve. At low loads, efficiency tends to be the same as near max loads (20% is about equal to 90%, depending on the power supply). Peak efficiency is approximately 40-50%, once you deviate either way, you start losing efficiency. Once again, this really comes down to the power supply, and you need to look up reviews to determine how the efficiency goes.

However, unless you're folding 24/7, or the entire time the computer is on at least, you probably won't get near 50-60% usage on your computer. So your average power consumption is probably 40-50% of your maximum power consumption. It really comes down to what you're doing on your computer and how much load you will be putting on it.