Is It Worth Investing In a High-Efficiency Power Supply? 328
MrSeb writes "If you've gone shopping for a power supply any time over the last few years, you've probably noticed the explosive proliferation of various 80 Plus ratings. As initially conceived, an 80 Plus certification was a way for PSU manufacturers to validate that their power supply units were at least 80% efficient at 25%, 50%, 75%, and 100% of full load. In the pre-80 Plus days, PSU prices normally clustered around a given wattage output. The advent of the various 80 Plus levels has created a second variable that can have a significant impact on unit price. This leads us to three important questions: How much power can you save by moving to a higher-efficiency supply, what's the premium of doing so, and how long does it take to make back your initial investment?"
The Maths (Score:5, Informative)
new efficiency @ load % - old efficiency @ load % = delta%
integrate over time (delta%*cost kw/hr) until result = new unit cost (solve for t)
Cooler (Score:2, Informative)
One advantage of a more efficient PSU is that it runs cooler. This is nice at least if you are going for a silent system, as less fans are then required.
Read this and decide for yourself (Score:3, Informative)
http://www.extremetech.com/extreme/143029-empowered-can-high-efficiency-power-supplies-cut-your-electricity-bill/ [extremetech.com]
Re:The Maths (Score:5, Informative)
Scenario 1: an always-on computer running near-idle for four years.
Idle power draw, 85% efficient PSU: 66 watts
Idle power draw, 80% efficient PSU: 70 watts
Delta: 4 watts
Total power difference over the four-year life of the computer: 140 kilowatt-hours.
At 5.5 cents per kilowatt-hour (cheapest power in the US), building with a more-efficient power supply makes sense if it costs no more than $7.70 beyond what the less-efficient power supply does.
Scenario 2: an always-on computer running Folding@Home for four years using both CPU and GPU.
Power draw, 90% efficient PSU: 215 watts
Power draw, 80% efficient PSU: 245 watts
Delta: 30 watts
Total power difference over the four-year life of the computer: 1.05 megawatt-hours.
At 36 cents per kilowatt-hour (most expensive power in the US), building with a more-efficient power supply makes sense if it costs no more than $378 beyond what a less-efficient power supply does.
The second scenario represents someone running F@H on a modern high-end computer in Hawaii -- not exactly "unrealistic".
Save more by buying small (Score:4, Informative)
A PSU has a power efficiency curve that looks like this [anandtech.com]. That article also explains what I'm about to summarize:
Pick a PSU that is no more powerful than you need, to keep your system in the middle of that curve, for maximum efficiency. 100% margin is more than plenty, so if your components will use 250W max, you don't need a 900W PSU. Look for something in the 500 range, or even less if you pick a good-quality PSU.
You probably won't be able to make a cost argument for maximizing efficiency, but you can build a quieter system focusing on efficiency, and it's quite satisfying obsessing over something different.
Re:Do you heat your house? (Score:2, Informative)
This is why "software engineer" is a term I will never use willingly. It is an insult to real engineers. Heat pumps do in fact put more heat into their hot side than they consume in work. They take heat from a low temperature resivoir and send it to a high temperature resivoir.
Re:The Maths (Score:2, Informative)
new efficiency @ load % - old efficiency @ load % = delta%
integrate over time (delta%*cost kw/hr) until result = new unit cost (solve for t)
You're missing the savings on removing that excess heat from your house too (in climates where that is relevant).
In a cold climate where you are heating your house, unless you can get better $/unit heating out of something else, the "waste" energy is heating the house anyway so it doesn't matter much.
In a hot climate where you are cooling your house, every unit of heat that you put into the house has to be removed. Firstly from the computer by making the fans work harder, then from the house itself by making your AC work harder.
Please revise your maths accordingly.
Re:More maths (Score:5, Informative)
That was true in the past when the PSU wasn't a particularly valued component and the industry standard method of rating their power output was 'think of a number, any number. Now write that number on the side.'
It's *less* true these days if you're buying from one of the decent brands. The numbers they write on their spec sheets actually bear some kind of resemblance to reality, these days: you can actually accurately spec up your expected draw against the capabilities of a PSU and expect it to more or less work out. It's worth leaving a bit of safety room, but you don't really need 2X.
Re:The Maths (Score:5, Informative)
Not to mention reduced heat output (and potentially less fan noise due to lower heat), important in many scenarios
Plus you have to add in costs due to the extra air conditioning load in the summer time (gotta remove all of that heat), and subtract in the winter time to account for the fact that your furnace needs to do less work to keep your house warm.
Re:Match Your Power supply to System Power Reqs (Score:4, Informative)
If you look at efficiency graphs, you'll see that power supplies are typically the most efficient under moderate load: at low and high load the efficiency drops. A typical desktop or home server is idle most of the time, so idle efficiency will have a big impact on the total efficiency. If you over-dimension your power supply, your idle load might be 10% or less of the max rating, which is far from the optimum of the efficiency curve.
I'd recommend getting a power supply that can deliver a bit more than what you need, for example 450 W if you think you need 350 W max. A bit of margin is useful since you might not have found the actual worst case or you might want to add components later. Also it avoids poor efficiency at the high side of the curve when the system is under load.
Re:It's not just the power (Score:5, Informative)
Newegg's calculator is a joke. It drastically overestimates requirements so they can pimp massive PSUs with higher profit margins. I suggest adding up the various component manufacturer specifications (i.e. max power draw of the MB, GPU(s), HDD(s), DIMM(s), and CPU(s)) and throw in 10-15 W for overhead, then buy a decent PSU with a load rated as close to that number as you can get. Even with a dual GPU setup, you are VERY unlikely to exceed 400W of DC power draw. My current mid-range single GPU system draws around 200W under load (gaming).
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Re:More maths (Score:5, Informative)
You're thinking of hardwaresecrets.com - they do the type of PS reviews only an EE truly appreciates! :)
Re:The Maths (Score:4, Informative)
600W is actually pretty enormous by modern standards. an i7 3770k will use 77W, a very high end GPU 250W, and motherboard and other bits about 50... so even for very very high end systems, you're talking about 400W total consumption with everything under maximum load. Under most normal usage you're talking more like 100-200W.
Re:The Maths (Score:2, Informative)
It doesn't stop the uneducated from buying 1000W power supplies.
I have a quad core i7 3.2ghz powerhouse with 2high end video cards and 4 drives in it and it's happy with the 400 watt power supply I put in it even at full load. Some people think bigger numbers are better, Others actually calculate what they need.
A while ago I built my new system using an el-cheapo 400W PSU I had laying around. The system was flaky under load, even though my components couldn't have drawn more than about 250W. I suspect voltage fluctuations even if it might have been able to supply the full 400W. I bought an upper-mid-range 600W PSU to replace it, which I have been very happy with it through the next two upgrades (which makes it about five years), I'm still using it in my gaming rig.
It might be overkill, but in the long run it was actually a very cheap investment.
Re:More maths (Score:4, Informative)
That brings home another benefit of picking a high efficiency power supply: generally a much higher quality and specs that you can actually trust. For instance compare the review of the Coolmax 750W [hardwaresecrets.com] with that of the Corsair VX450W [hardwaresecrets.com]. The el-cheapo 750W PSU blew up twice [hardwaresecrets.com] after they pulled just 500W while the 450W one managed to provide a stable 572W [hardwaresecrets.com] before it shutdown cleanly due to over load protection! So before buying a power supply it's worth reading a proper review of it, even if you only read the conclusion [hardwaresecrets.com] page [hardwaresecrets.com].
So just looking at much is saved on electricity is missing the big picture.