Slashdot Mirror
Is It Worth Investing In a High-Efficiency Power Supply?
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?"
new efficiency @ load % - old efficiency @ load % = delta%
integrate over time (delta%*cost kw/hr) until result = new unit cost (solve for t)
Sent from my ENIAC
we could probably use a computer to figure out the answers to those questions!
"National Security is the chief cause of national insecurity." - Celine's First Law
I tried to include an image of the formula using the IMG tag instead of text, but it wouldn't display. :( Any tips on how to include an image in a comment on /.?
I've never seen an image in a slashdot comment before, I think it's for our own safety.
Get your head out of your ass. Most electric heating is done with heat pumps. A heat pump pumps more heat into your house than the electric energy it consumes (that's why it's called that way). Heating by burning something is also more efficient than dissipating electric energy because you're cutting out conversion (see Carnot efficiency) and transportation losses.
And in the summer, if the AC is on, inefficient appliances make you lose double: once by consuming more electricity than they should, and a second time because the AC needs to consume energy to pump the heat out of your house.
The real embarrassment is that /. has never supported basic tags like <sup> which would allow proper math mark-up. Instead we get all manner of mangled, unreadable blobs for comments.
Slashdot gets worse every day... Pipedot: News for nerds, without the corporate slant
Perhaps the "pump" part of heat pump completely eluded you, since they do not defy the first law of thermodynamics as you seem to be implying.
Heat pumps work by having a sink source off of which they are pumping the heat from or away from. Most of the ones I know happen to be geothermal, which work because the sink which they are pumping from maintains a constant temperature year long underground. So, during the summer, the heat they can extract from that source would be cooler than the air above ground, but during the winter be hotter. They do this by extracting the heat from the source sink, rather than producing it themselves.
So in that respect, they work much like the fan does within your computer, since the air inside the case is much hotter when running than the air outside of the case. The fan can then displace that heat generated inside rather efficiently by just pushing the hotter air inside the case out, while bringing the cooler air from the room outside in without having to require an equal amount of energy to then power those fans as the equipment running inside of it, thus, like the grandparent, requiring less electric energy to power those fans than what the computer itself uses. If this were not so, then it'd make a lot more sense to completely seal computer cases, as the cooling benefit from the fans wouldn't make up for the amount of dust which they bring into the case during operation.
So the next time you're tempted to call bullshit on a well known physics principle, make sure you double check that you're not making some stupid mistake. Or else you'll end up looking rather foolish again when someone else points out how you don't know what you're talking about.
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.
I used to test server and PC power supplies for a living (until 2009). I do NOT recommend running at 50% load unless your PSU is a cheap turd and you are worried (rightfully so) about component failure. 80-90% load will give you better efficiency, a higher power factor, and less harmonics. Fyi, as a residential electricity customer you don't really have to worry about power factor or harmonics much but large companies can be charged by the utilities for abusing the infrastructure with a ton of shitty/under-utilized PSUs. Since the company I used to work for sold into enterprise, we were very interested in PSU performance and matching up components for efficiency.
At home, I run a decent 350W PSU now, and my system draws about 200W of DC power under load (i.e. gaming) with my components (single Intel 2500K CPU, 8GB RAM, ATI 7870 GPU. 1 HDD and 1 SSD) and around 130W when surfing the web or working. I literally couldn't find a decent, well priced PSU with lower DC power output when I built the machine 18 months ago. It cracks me up when I see guys putting 700W power supplies into their gaming rigs that never draw more than 300W (and none seem to understand the difference between AC power draw from the wall and DC power draw of the components in their system, which is what the PSUs are rated for). It's basically flushing money down the toilet in multiple ways.
Just my $0.02...
Disagree about peak efficiency. In my experience testing PSUs, it is normally found around 90% load. Newer PSUs have gotten a lot better and enhancing efficiency at lower load levels, but PSUs still work most efficiently when running near the load they are designed for.
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).
Yeah, I used to get the cheapest PSU I could. But after I somehow inexplicably fried some of my expensive components, like my GPU, I decided to drop in something a bit better.
When I dropped another $250 on a replacement GPU, I also decided to shell out real money for a nicer PSU and put my old PSU out to the pasture... in my kids' cobbled-together box.
Ended up going with a SeaSonic, since that's one of the brands that tend to be recommended by the Ars Technica Budget / Hot Rod box guide.
I wish I could find it, but there was some PSU snob site that went into all of the power benchmarking and provided pagefulls of data and charts like the other sites that benchmark CPUs and RAM. They managed to point out all the ways my old PSU was deficient and sorta almost turned me into a PSU snob as well.
While GP is woefully incorrect and you're right to call him out on it, your explanation isn't right either. Heat pumps can in fact pump against a gradient, and are mostly used to pump heat from a cold to a hot place. Air-source heat pumps (ie. coupled to the outside air rather than a geothermal reservoir) are used in parts of the US to heat houses in the winter and cool them in the summer. They're also what makes a refrigerator work. A fridge pulls heat from a cold place (inside the fridge) to a warmer place (outside the fridge). The resulting decrease in entropy needs to be balanced by an equal of greater increase in entropy, which is accomplished by converting electricity to heat. Or, to avoid the thermodynamic jargon, you're pumping against a gradient, so you need to spend energy to do so. The heat produced at the back of your fridge is the sum of the heat that was pulled out of the interior of the fridge + the heat-equivalent of the electricity the fridge consumed. This is also what an A/C does. Now, if we turn the A/C inside-out, so that it pumps heat from outside to inside, then you have the kind of heat pump we use to heat our homes in the winter. The sum of the heat that was pulled from outside and the heat-equivalent of the electricity the device consumes is larger than the heat-equivalent of the electricity alone, thus the pump brings more heat into your home than a resistor using the same amount of electricity. GP suggested to generate electricity from this heat gradient, but the flaw in his thinking is that the heat pump as well as any electricity generation device he can come up with are bound by the Carnot efficiency, so you can never get more electricity out than you put in.
How about reliability? I require a PSU that I know is going to
(1.) Not die within a year of running at 50-75% load
(2.) Not take any other components of my computer with it.
Power supply problems are the most annoying to diagnose, because the symptoms usually show up in other components (like apparent RAM corruption, HDD stuttering, etc). I would pay $50 extra for a power supply that is *not* 80-plus if it has stellar reliability, because it means I only have to build my computer exactly once. On that note, the Corsair HX series power supplies have not only stellar reliability, but also pretty much silent. I refuse to buy anything else, and you can usually them 20% off if you watch slickdeals.
Efficiency saves you money, while reliability saves you time *and* money. And time is a limited resource for some of us...
You're thinking of hardwaresecrets.com - they do the type of PS reviews only an EE truly appreciates! :)