some Power Supply questions

FireShadow

FireShadow

FireShadow

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6 Jun 2008
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hi

I am working on a new build and hoping you can help me understand power supplies better ...

-- question 1 --

psu efficiency is rated like "90% at 50% load"

which means efficiency will reduce as load increases ...

does this mean that running a 600w load through a 1200w psu
could be more much more efficient than a 600w load through a 600w psu ?

where is the sweet-spot?

-- question 2 --

can't find info about mobo and ddr3 power consumption ...

my estimates:

i7 930 @ 4.0 = 190w
gtx 460 = 170w
1 x ssd = 5w
2 x sata = 24w

asus p6x58d-e = ???
3 x 2gb ddr3 = ???

how do I work those out?

-- question 3 --

i will use this pc for work (as well as play) - it needs to be very reliable
otherwise i could lose income
that means I don't mind spending extra money if it increases reliability

which is the ultimate psu for reliability, and voltage protection etc.

at the moment I am thinking about:

Corsair Professional Series™ Gold AX850 High Performance 850W Power Supply
-- or --
Coolermaster Silent Pro Gold 1000W Power Supply

I know the 1000w is way higher than I need - but considering it might save me money in future and overall efficiency might be higher (700/1000 is closer to 50% load) - it could be a good idea

-- question 4 --

do power supplies draw current regardless of load?
because with transformers there is a high voltage circuit even when nothing is connected to the low voltage side ...

hope you can help me out
 
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Hi there, I'll try my best to answer your questions:

1)Yes, most power supplies are at their most efficient at around 50% load, but it varies between units - there are usually efficiency tests shown in PSU reviews at different load levels

However, the good ones are efficient at all load levels, and only dip slightly when under full (or very low) load. Look out for the 80Plus certifications, these ratings not only ensure the maximum efficiency, but also have required minimum efficiency levels at 25, 50 and 100% load.

2) I suggest using this power supply calculator.

3) With your spec i7 950/GTX 460 I would suggest the Seasonic X-650 as the PSU to go for, though the Corsair you mention is very good.

The seasonic is a good PSU and supposed to be reliable, but there is now way to guarantee that it won't fail (the 5 year warranty covers you for this, but it will take over a week to sort out). The only fail-safe option is to buy a second PSU, which you can install temporarily if the main one fails.

4) Do you mean when the computer is off or just idling (low load)? If it is idling then it will certainly draw current as it is still providing power to the low voltage electronics. When the system is "off" (ie you shut it down from windows) it will still draw current (the motherboard is still being supplied with power, waiting for the main switch to be pressed so it can spring back into life) - so if you want to save every last watt-hour of energy, turn off the PSU at the box or wall.
 
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Take a look at this

http://www.hardwaresecrets.com/article/Corsair-AX850W-Power-Supply-Review/1081

on all their reviews they check 20,40,60,80 and 100% load key data.

You should never aim to run a power supply at its max, some components will decay over time and therefore may not be able to deliver the 600w in 6 months. But realistically unless your likely to want to upgrade the machine and keep the supply there is no point in dropping under 50% or even close to 50% load.

Also bear in mind its not massively likely that your proc will be at max load and your graphics card at max load at the same time. Possible but not massively likely.

As far as getting good idea on load, loads of review sites such as Anantech, tomshardware run various test rigs and often show at mains power usage (remember at mains is higher as its before you lose 10-20% approx in the power supply). Find a similar spec being tested to get a good idea on actual mains draw (110V is also worse in that the % loss is higher than 220v btw).
 
Hardwaresecrets PSU reviews (as linked to by robskinner) are the best I've seen. They check the safety shutdown features work properly (without the unit going up in flames) and do full load and overload tests. Check out the load tests to see how efficiency drops off with load, and the overload tests reveal whether a PSU does what it says on the tin, or less, or even more. Some PSUs will shutdown bang on their rated output, and some quality units will deliver 100 or even 200W more staying within ATX spec. Those units could have been rated higher, but were essentially downrated in order to be awarded higher efficiency figures.

If reliability with no down time is really crucial to you, redundancy is by far the better solution than buying a 'Rolls Royce'. I.e. get two decent (but not ridiculous overkill) PSUs, give each a good testing/running in, then store 1 as a backup so you can immediately replace a failed unit.
 
hi, thanks for replies

the power supply calculator says:

i7 930 @ 4.0 @ 1.4v = 157w
mobo = 50w
3 x ddr3 = 9w

which gives my build total of 516w
add 20% for aging etc. = 619w
add 20% margin for psu = 750w

only thing is it can be cheaper to buy AX1200 today and never worry about having to buy new PSU in face of upgrades / sli / big video card etc.
 
My opinion on buying way more than you currently need (not just on PSU but everything really) is that you're just wasting money to carry unused capability along with you, eating into its life and warranty. Far better to buy just what you need and upgrade regularly when you genuinely need to, keeping your gear and your warranties new.
 
Buying an overspecced PSU for the long haul is a tricky one.
90%+ PSUs are fairly pricey, but are mainly newer tech so can demand the premium.

If you do the calcs you need to either use the PC at full load a hell of a lot or be aiming to keep the supply a hell of a long time to make the premium of a 90% return money vs an 80%.

I am sure 90% supplys will come down in price, the main diff is in the tech used, its nothing difficult but atm its not the normal, I bet in a few years all decent supplies will be up closer to 90% with no real price premium compared to now.

If this PC is "business critical" then overclocking to me is a no no, certainly not that much, from what I read other than in benchmarks real world you won't notice that mcuh diff. Is it really really worth the risk?

TBH that seems a high figure for that spec from what I see, unless as mentioned thats dual graphics?
 
must have slipped up on my calculator, new figures:

1 x gtx 460 = 170w
1 x i7 930 @ 4.0 @ 1.4v = 157w
1 x asus p6x58d-e = 50w
2 x sata = 24w
4 x fans = 16w
3 x 2gb ddr3 = 9w
1 x ssd = 5w
1 x dvd = 36w
TOTAL = 467w
add generous margin for error:
PLUS 20% for aging = 560w
PLUS 20% for psu = 672w

so a 650w psu will be adequate

oc's (similar) titan onyx only has 600w psu ...

anandtech's system power consumption doesn't make sense to me - only 330w
 
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