Power Supply Efficiency

Seasonic S12ii 430w power supply
January 22, 2010 at 19:24:18
Specs: Windows XP, Atom 330
Hello guy,

I am building a green home data server with the Atom CPU and Ion GPU and i want ask a question about power supply.

It is likely that my system will never get over 90Watt so, if i get a 430Watt power supply, will the large capacity affect the efficiency of the system? (The power supply is 80% efficient at 20% load)

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January 22, 2010 at 20:20:55
If u have 430w psu and the system use only 90w, the psu will provide only 90w, even if u have 1000w psu only 90w will be provided.

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January 22, 2010 at 22:44:05
What are you hung up on as far as the efficiency goes? The 20% loss in the functioning of the unit is lost predominantly to heat. All PSUs lose a certain amount of their input to heat in the conversion to the various outputs. How have you calculated your maximum load @ 90w ?

It is noted that the PSU you are looking at has two +12V rails.each rated at 17Amps. http://www.seasonicusa.com/S12II.htm
In the table listing the specifications of each model they all have two +12V rails supposedly rated at 17Amps yet the wattage "rating" for those rails varies from 288W ( combination of the two rails) in the S1211-330 up to 408W in the S1211-500

There is something screwy in their maths or methodology.
The 330 unit is only capable of carrying maximum 12amps on each of it's +12V rails not the 17 listed in their table.
The 430 model rates 15amps on each 12V rail. You need to be careful to balance your load across both rails if you proceed with buying this unit.

Does not inspire confidence in their product when the information they publish about it is flawed.

Latest thinking when it comes to powersupplies is to get one with a single +12V rail rated @ 30Amps or more so load balancing and peak demand no longer become issues.

Goin' Fishin' (Some day)

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January 22, 2010 at 23:44:44
As already mentioned, the computer will only consume as much power as it needs up to the maximum rating of the power supply.

Efficiency is a measure of the amount of power coming out compared to the amount of power going in. As stated above, all power supplies generate heat which leads to some power loss and a loss of efficieny. A power supply the generates less heat for a given power output is more efficient than one that generates more heat for the same power. A power supply that was 100% efficient would generate no heat at all. That is clearly not possible so there will always be some loss in efficiency.

80% efficieny at 20% load seems reasonable but where did you get these figure from. 20% load is not much, 80% load would be more useful. It is like quoting 50mpg for a motor car which is achievable providing you don't exceed 30 mph and keep at a constant speed with no stopping and starting.

Most power supplies are running at 70-80% load with the load constantly varying. As the load goes up the efficiency will come down until you reach the stage where the PS is running at maximum load and minimum efficiency. So a larger capacity PS than one that it really needed will be more efficient at a given load but the difference is not likely to be worth bothering with when you consider the difference in cost - unless you are buying a really cheap one then all bets are off.


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January 23, 2010 at 05:11:31
Thanks Everyone for you input. It looks like i didn't make my question clear.

Let me try again. 8)

Assume that my system's maximum power consumption is 90W and i have two choices of power supply.

Buying a 100W power supply & a 430W power supply.

The reason i want to buy the 430W, which has the capacity far exceeds the maximum load of my system, is so that i can use it for other systems if i want to. And since the price difference isn't a lot, it makes sense to get the higher capacity for the flexibility.

However, i want to know if the higher capacity would cause more energy waste due to extra components.

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January 23, 2010 at 08:09:52
There are plenty of options between 100W & 430W...250W, 300W, 350W, etc. Why did you narrow it down to just these two? Are you paying attention to the amperage specs or just wattage?

Getting the 100W PSU would be an extremely bad move as it would run at close to max capacity & would probably fail sooner rather than later.

The 430W unit won't sit there churning out 430W even though all that's required by the system is 90W...it will simply produce 90W, therefore it will be more efficient & run much cooler than the 100W unit.

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January 23, 2010 at 15:54:43
the 100W was just an example. I actually bought the Antec EarthWatts Green EA-430D. I chose that because of the efficiency (Bronze +), detactable 20/24 main connector, trusty brand, and price. Wattage and amps weren't the major issues as i knew i would never exceed with my current system.

Mainboard CPU+GPU = ZOTAC IONITX-F-E Intel Atom 330
5x 1.5TB hardrive (3x 7200RPM SG + 2 5900RPM WD)
1x SATA controller card.
3x 120mm fan.
Antec Three Hundred Illusion Case

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