Something I need a guru for?

[freewheeling]

I went through a big thing wherein my system was freezing shortly after I started using SageTV, and a number of "experts" on the Windows 7 forum told me I needed to buy a new beefier PSU. So I bought a 550 watt PSU and it didn't do anything. Then I bought a nicer 500 watt PSU and although I had some problems they eventually went away when I removed an old Belkin wireless card. But since I got this new Kill-A--Watt gadget I plugged it in to see exactly how much wattage my computer was drawing. Even under fairly intense tasks it rarely draws more than 90 watts and usually hovers between 70 and 80. The odd thing is that when I switch to the other computer using the KVM switch *it draws more current* and goes up to around 85 and becomes rock solid.

So, is this typical? Why do people think they need 400, 500, or 600 watt power supplies? I have the typical load devices such as cards, USB hubs, drives, etc. I'm building a new system and thinking about robbing the 500 watt PSU I have in this computer for use in the new one, and putting back the 300 watt. It seems like plenty, if this really is only using 90 watts at most.

[sic0048]

You'll need to list what hardware you have installed in the machine. But unless you have a ton of harddrives, or multiple high power graphics cards, etc, a 500 watt power supply is more than enough.

There are lots of PSU calculators on the web. Something like this. You simply put in what hardware you are using, press the calculate button and it will tell you what min PSU power you need.

The trick with PSUs is you definitely need enought power, but going too big is simply a waste of power and money.

[freewheeling]

Originally Posted by sic0048

You'll need to list what hardware you have installed in the machine. But unless you have a ton of harddrives, or multiple high power graphics cards, etc, a 500 watt power supply is more than enough.

There are lots of PSU calculators on the web. Something like this. You simply put in what hardware you are using, press the calculate button and it will tell you what min PSU power you need.

The trick with PSUs is you definitely need enought power, but going too big is simply a waste of power and money.

Well, that calculator says a minimum psu wattage of 34 watts. I tended to add everything I could think of, and said I had three DVD drives because 1 is also a BluRay reader. So I still don't quite understand why people seem to think they need 600 watt psus when under 100 is more typical. And I'm actually measuring 71 at the moment as I'm typing this. I've never seen it go above about 92.

I guess my point was that I have this neato 500 Watt PSU that I'd like to use on my new system, and was going to put the 300 watt unit that it came with back in. Since I'm also swapping out one of the DVD drives, I think the 300 watt would still be safe, and the 500 watt will keep my costs down. Then if I eventually get a hex core or something I can upgrade to a 600 watt and put the 500 watt back. But I prolly wouldn't need more than the 500 watt for a quad.

That Kill-A-Watt gadget is great if you want actual empirical evidence of what current you're using. I just plug it in between the surge suppressor and the computer and watch the display. It's been quite instructive. I figure the KVM switch must be drawing power from the computer that's NOT active at a given time, which is why when I switch to my other rig the power on this one actually goes up. Interesting. I'd never have guessed that.

[robogeek]

The wattage on the power supply is just one thing to consider when choosing a power supply. You'll also want to look at the amp rating of the 5v and 12v rails that will supply power to the motherboard, cpu, video card, hard drives, cd/dvd drives, and any other devices that plug into the PC. These ratings are printed on a sticker on the PSU. Figuring out what specs you really need for each voltage rail is difficult, that's why everybody always recommends higher wattage PSUs. The lower the wattage, the lower the amp supply to the electronics in the PC. The 12V supply circuit is usually the one that gets overtaxed and therefore you may need a higher wattage power supply just to give you enough amps on the 12V rail to keep everything happy. Then, if you have more than a few hard drives, you'll want a power supply that has at least two 12v rails so you can split the load of the hard drives between two different voltage regulator circuits instead of overtaxing a single circuit with a high amp draw. Typically it's the computer startup and hard drive spinup that will draw the most current, so you will want to see what the peak wattage draw is when the PC is first turned on and the hard drives spinning up to determine a bare minimum that is required to turn on the PC. And keep in mind, most PSU wattage ratings are probably overstated. Desktop PC with one or two hard drives and lower end graphics card can easily get by on a much less than 300W PSU. I think most cheap consumer home PCs are (or were) sold with 150W PSUs...I'm not sure if that is still the case with these newer multicore machines with higher end graphics that are becoming the new standard. A SageTV server with internal tuner cards and several hard drives probably shouldn't be run with anything less than a high quality name brand 300W PSU. But, nobody here will be able to tell you that it's going to be OK to switch to a lower wattage PSU based on Kill-A-Watt wattage usage. We'll need to know the exact specs of the PSU (the voltage/amp ratings on the rails that supply the internal components of the PC) and all of the specs of PC (motherboard, CPU, size, type, and # of sticks of RAM, hard drives, cd/dvd drives, graphics card, and any other cards or USB/Firewire devices pluged into the motherboard) the PSU is going in.

If both PSUs meet the minimum power requirements of your PC, then whichever PSU is the highest quality is the one you want to be using. You won't be saving much money in energy savings by switching power supplies, if that's your main reason for switching. As you can tell with the Kill-A-Watt, the PSU draws as much wattage as required by the PC not what the PSU wattage rating is. Going from a 500W to 300W PSU is a 40% reduction in PSU wattage, but it won't translate to anywhere near a 40% reduction in wattage used (if any change at all) when you check the usage of the 300W PSU with the Kill-A-Watt. It all boils down to whether the 300W PSU can supply enough amps on the 5V and 12V rails to power your connected devices without maxing out and amp draw on any of the rails. Going too low on wattage and cutting it too close will result early PSU failure which can send power surges that can burn out your motherboard, CPU, hard drives, and anything else that gets its power from the PSU. The few dollars a year in energy savings or the $50-$100 you saved on a no-name brand PSU could cost several hundred of dollars a year or two down the road if you need to replace the PSU, motherboard, CPU, and some hard drives due to PSU failure...I've been there and done that, it's not cool

It's been my experience that you get what you pay for. A higher priced, higher quality 300W PSU can easily outlast a dirt cheap 500W PSU. But even a higher quality lower wattage PSU is going to be prone to early failure if you're asking too much from it. Too many hard drives starting up and running on a single 12V rail will most certainly lead to early PSU failure.

[sic0048]

Originally Posted by freewheeling

Well, that calculator says a minimum psu wattage of 34 watts.

I don't think you pressed the "Calculate" button after inputting all your hardware. It shows 34 watts without inputting anything. This calculator doesn't seem to recalculate after each selection.

[jpwegas]

Originally Posted by freewheeling

The odd thing is that when I switch to the other computer using the KVM switch *it draws more current* and goes up to around 85 and becomes rock solid.

I not sure why it would be drawing more current when you switch to the KVM.

But whatever the reason, more wattage means more heat. In my experience when electronics get more stable when they get hotter it means some part is borderline failing. How old is the motherboard?

--John

[Fuzzy]

Originally Posted by jpwegas

I not sure why it would be drawing more current when you switch to the KVM.

But whatever the reason, more wattage means more heat. In my experience when electronics get more stable when they get hotter it means some part is borderline failing. How old is the motherboard?

--John

I think he was referring to the power draw stabalizing at 85W. regardless, the KVM switching away may inadvertantly be causing the video card to ramp up to full speed. I've seen video cards do strange things when they have no monitor attached, and that would be my best bet.

[freewheeling]

It's actually a Gateway that's less than a year old. (Probably more like six months.) It came with the 300W power supply and I only changed it on the advice that the power supply might be causing it to freeze. However, I'm pretty sure now that the cause of the freeze was that Belkin wireless card. It had some kind of glitch, and as soon as I took it out it stopped freezing.

I have a Pentium dual 2.5Ghz, with two DVD drives and three hard drives (one green and two low power) 1TB (the green one), 640GB, 500GB. I use half the 1TB and the entire 500GB for SageTV. The 640GB drive is used to back up my system partition. I don't bother to back up my TV files. I also have a 240GB USB drive connected, but don't use it much. I have the HDPVR plugged into another USB, as well as the USBUIRT on a hub. A modem card that I don't use. I may get another PCIexpress SATAII card, since I only have four SATA connections on the MoBo and five devices. (Currently not using one of the DVD drives.) I also have a USB wireless adapter plugged in. That's it.

Here's the layout for the "rails" on the 300Watt LITE-ON PSU that came with the board:
+5V/20A MAX
+5V(SB)/2A MAX
-12V/0.5A MAX
+3.3V/20A MAX
+12V1/10A MAX
+12V2/30A MAX

I also have a cheapo 550 Watt PSU but am afraid to use it. I got it for free. After Newegg sent it to me on a mistaken order they didn't even bother to have me send it back, just refunded my money. It can't be very good.

I'll restart, and note the peak wattage use. [Update: Peak wattage was 111]

[Spectrum]

Another thing that is not highly publicized about power supplies but has been shown in research lit is that most are horribly in-efficient below ~60-70% of their max rating. One research project I have seen used multiple consumer grade power supplies in servers rather than 1 or 2 2.5kW supplies and they were able to reduce overall energy consumption by using multiple power supplies and staging them in and out based on current and projected future load.

Also using something like a kill-a-watt or watts up is only going to show you power usage averaged over a pretty substantial period. I'm not sure what they sample at but I can guarantee you it is not high enough to see short spikes. In the lab I worked in for awhile in grad school we had a power meter that took 10,000 samples per second. Granted a quality power supply should be able to handle a spike of 1.5-2x its max rating for a very short duration, but if you have spikes on a consistent basis your supply you will burn it out much sooner.

[freewheeling]

Originally Posted by sic0048

I don't think you pressed the "Calculate" button after inputting all your hardware. It shows 34 watts without inputting anything. This calculator doesn't seem to recalculate after each selection.

Right. It was 84 watts when I clicked recalculate, which is about right I guess. Between 80 and 90 on the Kill-a-watt meter, although it spikes up to 124 during boot. Way short of 300 W though.

[panteragstk]

For an htpc a 300 watt psu is fine unless you start putting in a lot of drives and use it as a server. Then you start talking quad core cpu's and multiple tv cards and the wattage goes up. Granted the 400watt psu in my htpc will never see anywhere close to that wattage, but there is nothing wrong with overkill.

Now, my gaming pc is another story. Multiple power hungry video cards and an overclocked cpu make power consumption shoot through the roof. I'm using close to 500watt on average.

[sic0048]

Originally Posted by freewheeling

Right. It was 84 watts when I clicked recalculate, which is about right I guess. Between 80 and 90 on the Kill-a-watt meter, although it spikes up to 124 during boot. Way short of 300 W though.

Nice to see it reinforces the "real time" results you see. I think it certainly proves that the 550w PSU is way overkill in this situation.