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This is something I've been thinking about recently and I have yet to find it covered anywhere.

I know that having a power supply that isn't over-rated for your purposes will generally increase efficiency (very slightly), but what effect would an over-rated PSU have on heat output, "stress" on the PSU and overall longevity vs one that is lower wattage on the same system?

For example, I've been picking up "used" (brand new pulls, never been turned on) Seasonic power supplies on eBay for several years now. I don't know what systems they are being harvested from, but they're an unmatched value for most systems. I get 350W 80plus (white or bronze) units for $10-$15. All they have are scratches from being installed and removed from a case. No dust... still smell like new electronics. I recently snatched up a 10 pack of 550W Seasonic 550HT units (80plus white) for $135 shipped. It was an absolute steal! They're the black models with sleeved cabling, and they too are totally unused aside from installation scuffs.

Given the two options of power supplies I have now, I'm trying to figure out if upgrading the super cheap 350W units to 550W units would make them run cooler, run quieter or last longer. I have found myself slowly building up the gaming chops of my other systems, so their 350W PSUs (Seasonic 350ET mostly) could be a limitation if a more powerful graphics card is added at some point... but if it doesn't ever come to that, is there any reason NOT to swap out a 350W for a 550W of similar design and efficiency? The 350W units will be easier to install as replacements in other systems for family members and friends (far less cabling), so I'm inclined to get as many of my "keeper" systems running these 550W supplies as possible, I just wondered if anyone else has come across this before.

tl:dr: If you have a 550 and a 350 watt power supply, both of similar design and efficiency, will the 550 be under less stress and run cooler, and possibly last longer under the same moderate to low load? Most of these systems idle in the 50-75W range and maybe peak in the 200W range.

ozzuneoj wrote:

tl:dr: If you have a 550 and a 350 watt power supply, both of similar design and efficiency, will the 550 be under less stress and run cooler, and possibly last longer under the same moderate to low load? Most of these systems idle in the 50-75W range and maybe peak in the 200W range.

If it's constantly under operation (not being turned off and on), and at the same operating temperature - generally yes, the higher power supply will last longer.
It's almost completely about heat death of the electrolytic capacitors under that condition, and larger supplies can handle more current and the resulting heat over time (..in fact longevity is often rated at a steady 50% rated voltage output).  Of course, it could be that the *quality* of the smaller power supply's capacitors are just better in a particular unit and will last longer, or the overall design of the power supply might just be better - so it's only a general approximation. In that estimate you also have to consider the number of capacitors and one or more of the cap's failing prematurely.

Higher wattage PSUs will definitely be more durable, but baseline reliability is probably a bigger deal if you're not stressing them much anyway.

Capacitor lifetime is all about heat. Loading them with ripple current mostly just wears them out quicker because of the internal heat it generates. Capacitors in a PSU will probably see similar amounts of environmental heat regardless of the PSU's power rating, but the bigger caps needed to get that higher rating will probably have more surface area with which to dissipate their own heat, and they'll probably generate less heat of their own due to lower internal resistance. Having more overbuilt semiconductors is probably worth something, but that's a lot tougher to quantify than anything about caps.

If peak loading is <2/3rds of the rating and Seasonic sees fit to put their name on it, it probably doesn't matter, especially in the short term. I'd probably be more trusting of one of the 550W parts to last a decade without issue, but if a system is idling at 50W, it'll probably be significantly more efficient doing so through one of the 350W units. That's probably your main trade-off right there.

Looking at your system specs I'd say your likely to be using around 250 watts when gaming.
All Seasonic PSU's are generally well engineered and capable of 120% their rating
not like cheap units that are only 50%.
Power usage between the two would be barely noticeable- The 550Watt is likely to
last longer-and another bonus,it should be quieter............................................

For 200-250W max usage (output) I wouldn't worry about using a decent 350W PSU.

I'd go for whichever is quieter. If power usage matters, measure the power use of them both at idle/load and take the results into consideration.

The speced longevity of capacitors is at the rated temperature, 85C or 105C, and is doubled for every 10C drop.
The specs are usually a few thousand hours. About 2000-8000 hours for wet electrolytics, and more for polymer ones (those usually with a visibly metallic case).

Thanks for the info guys! I was basically suspecting that The higher wattage units would be built "tougher" in general. Obviously there's no simple way to quantify this aside from looking at all the components internally... But it's good to know anyway. I was trying to figure out if the slightly lower efficiency I'd be getting would actually cause there to be more heat or stress on the higher wattage ones.

Also, my main system won't be using one of these. I have an XFX 550W 80Plus Gold unit... Also built by Seasonic.

I will certainly be testing the power consumption of my media center PC with the 350 and the 550 when I make the switch, and I'll report my findings here. It is running an Asus B85, G3220 and 750 Ti. It definitely doesn't need the extra power but it doubles as a LAN gaming system frequently and is likely to get a CPU and GPU upgrade at some point down the road.

We can make assumptions based on brand and such, but it's fairly difficult to quantify the "quality" of a PSU without a knowledgeable person doing a complete teardown and assessment.  Even under the same brand, models from different product lines can often be completely different animals. If we assume that they're of equal quality and design, then yes, I would expect the larger unit to last longer, as it has larger capacitors and is under less overall stress for a given load.

At the same time, if they're both high quality units and loads are pretty light, the difference in actual lifespan might not be meaningful (or come at a point where the rig has outlived its usefulness).

In your case, while the massive factor of safety in wattage may not be decreasing your odds of failure by any measurable amount, you're getting reputable brand name PSUs and that's good.  Even better that you're getting them for a good price.  Do keep in mind that capacitor ageing is real, and like all things, the older it gets, the more likely it is to fail.  I wouldn't have a problem buying those PSUs for that price if they're <5 years old, but if they're much older than that just get a new one for $25.

Heat output of a PSU is a product of the inherent inefficiencies in electronics.  If you have an 80% efficient PSU and your system wants 100W, then the PSU is producing 25W of "waste" energy/heat.  Assuming both a 250W and a 1000W PSU are 80% efficient at this load, they'd both generate the same amount of waste heat.

All PSU's follow a roughly equivalent Efficiency Curve whereas they're generally most efficient at 50% of their rated peak output.  Higher quality PSUs will tend to have a flatter curve (smaller difference in efficiency over the output rang) whereas low quality PSUs' efficiency can drop off quite dramatically above/below the 50% range.  The 80 Plus Certification standards have done a lot to improve both overall efficiency, as well as made PSUs' efficiency curves more consistent.

As a rule of thumb, you'll want to determine the wattage of a system running at typical load (gaming is a good test) and buy a PSU whose wattage is about double that figure.  As you said correctly, having more watts than you need is beneficial because it stresses the components to a lesser % of their capability which should allow a PSU to last longer.  However, keep in mind that if you're following the typical guidelines, your PSU should already have a (roughly) 2x factor of safety already and there's definitely diminishing returns in play.  Buying higher wattage PSUs is not a good substitute for buying higher quality PSUs.

While a 750W PSU can power a system that only draws 100W under load, it is doing so at 14% of its rated capacity....and that's the most the system is going to draw.  You're operating at the absolute worst point in the PSUs efficiency curve (in today's 80 Plus Bronze PSUs that may be only 5-10% less efficient than at 50% loading).  The bigger issue is pricing.  Because it takes more/beefier components to output 750W than it does to output 350W, higher wattage PSUs cost more than lower wattage PSUs.  If your system only draws 100W, why not spend that extra money on a (ie 80 Plus Gold) 350W PSU so that you're gaining even more efficiency (maybe >90% efficient vs 80% on the CX750) at your typical operating loads AND the internal components are higher quality.

Lastly.  Don't forget how many OEM machines (Dell / HP) are out there running on low wattage, no-name PSUs for 10+ years without problems.  Yes, failures happen and you can decrease your odds of a failure by buying high quality aftermarket PSUs, but nothing grants you immunity.

Related question:  I wonder if voltage regulation is consistent across the entire operating range of PSUs.  Obviously many/most modern PSUs can support the Intel low-power sleep states, but it seems like a theoretical 2,000W PSU might have a looser voltage regulation while producing 50W than a 200W PSU.

Just wanted to post an update.

I decided to swap out the Seasonic 350ET from my HTPC\Gaming system (currently G3220, 2GB 750 Ti, SSD, HDD... eventually will have an i5 of some flavor) for one of the 550HT units, to see what effect it would have... and it seems that efficiency was impacted as expected, but not by much. At idle, maybe 3-5 watts (~41 to ~46) and during a moderate gaming load (Minecraft with high draw distance and 2x render resolution at 1920x1200) it was fluctuating within the same range with both units, possibly 2-3 watts higher with the 550HT (~137 vs ~141). This is a really rough and non-scientific test, but its obvious that doing this isn't going to kill my electric bill. Especially since this system is probably used less than 10 hours per week unless we have a LAN gaming night.

Presumably, the difference will be even lower when this system is eventually upgraded to a beefier CPU.

I will say, the only other negative I can see in doing this is that the 550HT has a massive amount of wires compared to the 350ET, and they can be a bit of a chore to hide in a smaller case.

Still, I like knowing that if I need to troubleshoot a video card or even if I stumble across a really nice card for a great price, I can simply drop it in this system with no worries about powering it safely.

I'm also not concerned about power supply age with these. I know that capacitors do change over time, but being a fan of vintage electronics and computers, I use hardware that is 15-30 years old on a daily basis. Aside from the capacitor plague of the early to mid 2000s, I rarely see failed capacitors or capacitors causing problems of any kind (tantalums on 20+ year old stuff, and some aluminum SMD caps from the mid 90s can have issues). Garbage power supplies (like Bestec) are an exception though... they may seemingly work fine for 5-10 years or more, but they likely have swollen caps if you open one up. On the other end of the spectrum, I have an IBM 5150 from 1987 with the original 63.5W PSU and it has no swollen caps and runs fine, as well as various AT and ATX power supplies from 1993-2000 with no swollen caps or voltage problems... and most of those were used for many years. Anything can fail, but I don't think age has anywhere near the impact of quality of components, manufacturing defects and extended exposure to heat.

I would prefer lower wattage that could allow more durability and longevity. Hopefully, my current power supply for my desktop could really last longer than I expect..

Wow, you bumped an almost 1 year old thread.

As an update, this system is now running an i7 4790 and GTX 1050 Ti. So, it still isn't a power hog by any means and likely would still be happy with the 350W unit, but I'm comfortable having the extra leg room of the 550W.