Using Kill-A-Watt for PC Wattage

[allenk09]

I just bought a Kill-A-Watt at goodwill for $1, and I'm reading off of it that my computer is pulling under 400W at full load (video and CPU) with a Phenom II X6 OC'd, and a GTX 670.

Is this representative of what power supply wattage I need, or am I looking at something wrong? I would really like to go to a lower wattage power supply as these high power 750W+ ones are seemingly disturbingly not part 15 compliant, and as a radio operator that really hurts my radio capabilities (I've been through 3 750W power supplies, all noisy on RF).

 

[wirednuts]

really? i never noticed that... thanks for the info.

if its pulling under 400w at full load- then a 400w power supply should be fine. that reading youre getting is before the power conversion happens from ac to dc, so its likely you can knock 15% off that to get what your computer components are eating. which if you buy a power supply and want to run it at the ideal 80% load, a 400 or 500w is what you want. just make sure its name brand and youre getting a real 400-500 watts.

 

[postmortemIA]

You do not want full load on PSU. Probably not more than 80% load. The efficiency of PSU is not the best at full load. Also there are many ways to specify Watts of PSU, and there are many ways to measure it. One takes into considerations power factor. Another looks at input consumption. So, in short, if your max consumption on input is 400W, I'd be looking at least 500W PSU.

 

[allenk09]

You do not want full load on PSU. Probably not more than 80% load. The efficiency of PSU is not the best at full load. Also there are many ways to specify Watts of PSU, and there are many ways to measure it. One takes into considerations power factor. Another looks at input consumption. So, in short, if your max consumption on input is 400W, I'd be looking at least 500W PSU.

Yeah it's actually under 400W. I'll try some 500W supplies. It's a lot cheaper buying hit or miss (RF wise) 500W supplies than 750W ones. Thanks for the info.

just make sure its name brand and youre getting a real 400-500 watts.

The name brands are actually the ones I've been getting @ 750W. I have a $15 350W diablotek that's been powering along for 2 years and is clean as a whistle on the RF spectrum.

 

[allenk09]

Idle: 137W
Prime 95 Blend: 271W
Prime 95 Large FFT: 295W
GUIMiner Full GPU @ All Cores (All cores not 100%): 270W

It almost seems as if there's something wrong with the numbers. It was a brand new Kill-A-Watt still in box with the plastic screen cover on.

 

[wirednuts]

the numbers look right to me. most people dont realize how overkill their power supplies are. peak efficiency is usually around 80% load, but there isnt much loss at 50% either and a lot of people like to run around that range to ensure the supply lasts a long time

i think your setup calls for a 500w max... 750 is definitely a waste.

 

[bryanl]

Why do 750W+ PSUs emit more EMI when they have L-C filters like those in lower rated PSUs?

 

[allenk09]

Why do 750W+ PSUs emit more EMI when they have L-C filters like those in lower rated PSUs?

I'm not sure. Both my Corsairs and Antec are absolutely ridiculous. I wouldn't believe for a second they pass FCC part 15.

Here's the old Corsair TX 750. All the squiggly lines are the power supply.

It SHOULD look like this. You can clearly see man made, time interval signals with a plain dark blue background...but no squiggly lines. (This is actually with the diablotek)

 

[John Tauwhare]

Idle: 137W
Prime 95 Blend: 271W
Prime 95 Large FFT: 295W
GUIMiner Full GPU @ All Cores (All cores not 100%): 270W

It almost seems as if there's something wrong with the numbers. It was a brand new Kill-A-Watt still in box with the plastic screen cover on.

Those numbers are right. Mine:
Idle 1.9G: 105W
Idle 4.8G: 137W
Prime 95 Small FFT: 300W
Heaven 4.0: 405W
3DMARK11 @5.0G: ~500W

 

[jaqie]

What you want to do for a power supply in this situation is:
check what efficiency your current power supply is. If it is not 80+ then you aren't actually supplying a lot of that wattage to the PC.
Find out about how much the PC actually draws when running prime95 in max heat mode along with furmark in ultra max burnin mode (or whatever they call it).
prime95 runs cpus harder than any normal workload, so take these figures as the max your system will draw. Take that figure and find out how much the computer is actually using out of your current wall draw, again this has to do with the power supply's efficiency rating. That is the max draw you need to worry about. Add 20% to that figure and look for a power supply that is right around that, but it must be rated at working temperature and not some crazy low number like 20c or so. Last I checked only the really good PSU manufacturers do this.

My system is a phenom II thuban x6 oc to 3.5GHz all bios power saving turned off, default voltage.... with a factory OCed gigabyte HD6850 1GB, an SSD, HD, and two DVD drives along with an intel 1000/GT pcie card and an M-audio 2496 audiophile sound card and my PC draws ~380W out of the wall in max drain mode out of the wall with a pc power & cooling 420w 80+ white label (they were sold before the gold, silver, bronze, et al came out) so I am just about perfectly sized PSU for my load.

 

[VirtualLarry]

my PC draws ~380W out of the wall in max drain mode out of the wall with a pc power & cooling 420w 80+ white label (they were sold before the gold, silver, bronze, et al came out) so I am just about perfectly sized PSU for my load.

Considering PSU aging, I personally wouldn't have cut it so close. I would have gone for a 500W, or possibly a 650W for future expansion.

 

[2is]

If KaW is reporting 400 watts you're actually drawing less than that. KaW is reporting draw from the wall which also includes PSU inefficiency. The draw from the power supply (which is how they are rated) will be even less. But as stated, you don't want to cut it too close. I wouldn't call 420 watts on a 380 load the perfect PSU personally. Something as simple as plugging in an ipad can increase power consumption by 10 watts.

 

[jaqie]

Considering PSU aging, I personally wouldn't have cut it so close. I would have gone for a 500W, or possibly a 650W for future expansion.

If KaW is reporting 400 watts you're actually drawing less than that. KaW is reporting draw from the wall which also includes PSU inefficiency. The draw from the power supply (which is how they are rated) will be even less. But as stated, you don't want to cut it too close. I wouldn't call 420 watts on a 380 load the perfect PSU personally. Something as simple as plugging in an ipad can increase power consumption by 10 watts.

You must remember this is a white tag 80+ supply meaning the actual draw from the components is quite simply far closer to 300w, almost 300w on the dot.

Now take a look, 300w absolute max draw, 420w power supply rated by pc power & cooling specs (which rate at 50c working temp, mine is pre ocz merger)... and I have some nice headroom. That's over 25% headroom or 120w at 50c internal psu temp.

This PSU was purchased ~6 years ago now, and is still running cool until I run the tests mentioned above. It was an infrastructure investment back when I had a core2 duo and a gts250 video card. The power draw was lower in total, and I have been able to upgrade without having to worry about the psu. I prefer quiet systems, which means low power draw video cards with really good HSF are always a priority to me, so for me this is the perfect psu. I will probably replace it with a 500w pc power & cooling mark III in a few years simply to have the 8 pin p4_aux 12v connector instead of the 4 pin it has. The wattage is still perfect for me, given my needs and wants.

 

[lakedude]

I just bought a Kill-A-Watt at goodwill for $1, and I'm reading off of it that my computer is pulling under 400W at full load...

What are you running to generate a "full load"? What do you normally use your PC for?

The most I can make my PC draw is about 265W running the OCCT "power supply" test and that took some tweaking. If you just let the test run without tweaking the CPU will be overloaded and bottleneck the GPU.

My system (i7-3770s with HD 7850) pretty much always stays under 200W under any normal (non-stress test) use.

Anyhow power supplies function most efficiently in the middle of there range. Draw too much or too little and they become less efficient.

If your system draws 400W during a stress test it is likely to draw perhaps 300W during normal use. If this is the case a 600W PSU would be good for efficiency's sake. A 500W or 550W should cover it. A 400W might work but that is cutting it too close IMO. There isn't any need to go over a 600W or 650W.

the numbers look right to me. most people dont realize how overkill their power supplies are.

Agreed

750 is definitely a waste.

Agreed

 

[bryanl]

I'm not sure. Both my Corsairs and Antec are absolutely ridiculous. I wouldn't believe for a second they pass FCC part 15.

Here's the old Corsair TX 750. All the squiggly lines are the power supply.

It SHOULD look like this. You can clearly see man made, time interval signals with a plain dark blue background...but no squiggly lines. (This is actually with the diablotek)

Is that due to emissions coming out the AC cord side or the DC outputs?

 

[Charlie98]

I've got a KaW and it's a lot of fun... My desktop 750w PSU is supreme overkill, I'd be lucky if I saw 200w in normal day-to-day loads... 25% of the PSU rating. I thought about downgrading it, but I have it already...

For that matter, my HTPC's 430w is overkill by about the same factor. Oh, well, live and learn. Knowing now what I didn't know then...