We dont have the power, capt'n!

[Linias]

I don't ever want to hear my computer call this out in a scottish accent.

Is there a danger to buying too much power? I.E. if I buy a 750 watt, and I am only drawing 500; will a good PS scale down? Or is it going to eat 750 no matter what?

My ideal PS would only drink what it needs from my wall socket, but be capable of openning the firehose if it needs to. Also, it should only be a noisy as it's current requirements make it. (I.e. if it's not doing much, it should be nice and quiet).

I have built my own PC before (this is my third), but tradionally I have way overpowered my machines. I don't even know how much I need. I am thinking of:

* ASUS Maximus IV
* unlocked i& sandybridge (don't know which one yet)
* maybe 8 gig or so RAM?
* 2 Harddrives
* 2 DVD / Blu-ray drives
* Single video card (undecided on which). Not likely to go dual
* Water Cooled. Probably just one loop (? Do I need two?)
* some associated lights and fans. I like my lights

how much power should I be buying for? Will it have enough room if I add in some other stuff? I don't want to buy a new PS just because I decided to throw another fan in. I also don't want a wind tunnel in my office to power a web browser.

[xr4man]

the power supply rating is what the maximum power it can safely put out. that does not mean that it puts out that much power all the time. the power supply can't push out current. the current has to be pulled out by your components. so if you have a 750w power supply and you are using a nice new amd fusion mobo, you'll only be PULLING about 35w from the power supply.

understand?

as for how quiet it is, that is up to the quality of the fans used in it. i haven't looked, but maybe you can find a power supply that has it's own fan controller? i know i had one that had a knob to adjust fan speed, but i don't know if there are any "smart" power supplies that can do that on their own.

i've found power supply calculators on line that can help you determine your power needs so you don't have to waste money on buying an overly large PS.

just found this one on newegg. looks simple to use.
http://educations.newegg.com/tool/psucalc/index.html

[SXRguyinMA]

you always want more power than you'll actually use, that way the PSU will be running closer to it's peak efficiency (which is nowhere near 100% load) and it'll last longer and run cooler as well.

[xr4man]

yeah, i should have mentioned that as well. if you calculate that you only need 306w (that's what the newegg calculator just told me) you actually want to go well beyond that. for some reason the number 70% is sticking in my head. so you want to have your power demands be less than 70% of what the power supply can put out. i could be wrong about that percentage though.

[Linias]

Okay. So say it comes up with 500w calculated. There is no "downside" to buying a 1K, other than price.

Thank you all for the advise!

[SXRguyinMA]

none at all, if your rig willa ctually be using 500W, then the PSU will be running at 50% load most of the time, and most PSUs have their highest efficiencies at 50-70% load, so you'll be good to go

[xr4man]

ahhhh, there's that 70% i was thinking about.

[slaveofconvention]

Just one more variable to throw into the mix - it's generally accepted that PSU capacities decrease with age - I'd also account for a 10% drop in capacity per year - if you buy a 1k, assume it'll be closer to 900w after a year, 810w after two, 729w after 3 etc. As previously mentioned, trying to keep your system at or under 70% load will help too, so if you assume the above, and stick to the 70% thing, then IF you want to be able to safely and reliably supply a 500w system for several years to come, you might actually NEED the 1kw, but bear in mind all of those figures really are a worst case scenario - the PSU aging effect is MUCH slower in systems where the PSU is not working close to capacity....

Bottom line, buy the best PSU you can afford with a peak rating of at least 150-200% of the recommended requirements shown in one of those online calculators...

[x88x]

Just one more variable to throw into the mix - it's generally accepted that PSU capacities decrease with age - I'd also account for a 10% drop in capacity per year - if you buy a 1k, assume it'll be closer to 900w after a year, 810w after two, 729w after 3 etc.

I'm not sure those numbers are right unless you buy a really crap PSU, but yes, capacitor aging does affect the current levels a PSU can put out. I believe how much also depends largely on the load that they have been under during that time. My personal favorite online PSU calculator actually has an option to factor in capacitor aging.
http://extreme.outervision.com/psucalculatorlite.jsp

Regarding the noise levels, most quality PSUs these days have temperature controlled fans that will only run as fast (and loud) as they need to in order to keep the PSU components at optimal temps.

Another thing to consider is the efficiency rating of the unit. This determines how much power is lost to waste heat/etc from the wall to your components. Most decent modern PSUs will be at least 80% efficient most of the time, but a lot will operate much better than that. Check for "80 PLUS" certifications on the units you consider. Currently I only know of one 80 PLUS Platinum unit, and it's (iirc) a 550W unit, and is very expensive for that power bracket. 80 PLUS Gold are more common, but still quite expensive. I don't remember seeing 80 PLUS Silver units very frequently. 80 PLUS Bronze and 80 PLUS are the most common. Newegg has an 80 PLUS filter in their PSU section, so that should make finding one of the level you want easier.

Another factor to look out for is PFC. Make sure you get a unit with active PFC. Basically, this controls the AC-DC converter input signal to make everything past that operate more efficiently.

Last, once you find a unit you like, check for a review of it on HardOCP and jonnyGuru. They both do really in-depth analyses of PSUs.

[Undead]

Just one more variable to throw into the mix - it's generally accepted that PSU capacities decrease with age - I'd also account for a 10% drop in capacity per year - if you buy a 1k, assume it'll be closer to 900w after a year, 810w after two, 729w after 3 etc. As previously mentioned, trying to keep your system at or under 70% load will help too, so if you assume the above, and stick to the 70% thing, then IF you want to be able to safely and reliably supply a 500w system for several years to come, you might actually NEED the 1kw, but bear in mind all of those figures really are a worst case scenario - the PSU aging effect is MUCH slower in systems where the PSU is not working close to capacity....

Bottom line, buy the best PSU you can afford with a peak rating of at least 150-200% of the recommended requirements shown in one of those online calculators...

@_@

Capacitor aging is a non-issue with high quality PSU's. You can expect to see a "good" unit lose about 10-50w over the period of about 5 years. More if you have a unit with poor internals.

the power supply rating is what the maximum power it can safely put out. that does not mean that it puts out that much power all the time. the power supply can't push out current. the current has to be pulled out by your components. so if you have a 750w power supply and you are using a nice new amd fusion mobo, you'll only be PULLING about 35w from the power supply.

understand?

as for how quiet it is, that is up to the quality of the fans used in it. i haven't looked, but maybe you can find a power supply that has it's own fan controller? i know i had one that had a knob to adjust fan speed, but i don't know if there are any "smart" power supplies that can do that on their own.

i've found power supply calculators on line that can help you determine your power needs so you don't have to waste money on buying an overly large PS.

just found this one on newegg. looks simple to use.
http://educations.newegg.com/tool/psucalc/index.html

Actually, all "good" PSU's are rated at their continuous output. There are alot of poor PSU manufacturers that use the peak rating on the label, though.

And also, the PSU pulls AC power from the wall socket, and it then filters it, and converts it to DC power, which is then fed to the components.

Lastly, all PSU "calculators" are junk because they grossly overrate the amount of watts you need.

you always want more power than you'll actually use, that way the PSU will be running closer to it's peak efficiency (which is nowhere near 100% load) and it'll last longer and run cooler as well.

Your statement only makes sense if you run your PSU at 50% CONSTANTLY. A general user will have a vast amount of loads ~ when browsing, benching, gaming, folding and what-not. Buying much more watts than you actually need is a waste of money plain and simple, no matter how much people try to justify it.

I have built my own PC before (this is my third), but tradionally I have way overpowered my machines. I don't even know how much I need. I am thinking of:

* ASUS Maximus IV
* unlocked i& sandybridge (don't know which one yet)
* maybe 8 gig or so RAM?
* 2 Harddrives
* 2 DVD / Blu-ray drives
* Single video card (undecided on which). Not likely to go dual
* Water Cooled. Probably just one loop (? Do I need two?)
* some associated lights and fans. I like my lights

how much power should I be buying for? Will it have enough room if I add in some other stuff? I don't want to buy a new PS just because I decided to throw another fan in. I also don't want a wind tunnel in my office to power a web browser.

550w is able to power any single GPU setup currently available.

Is there a danger to buying too much power? I.E. if I buy a 750 watt, and I am only drawing 500; will a good PS scale down? Or is it going to eat 750 no matter what?

A PSU only draws the amount that is required for your rig.