Is UPS Power Draw accurate?

[xazraelx]

I keep reading posts about which power supply we need, and I'm currently running the build in my sig, but my UPS says I'm pulling 180w idle, 252 under full load running Prime95 stress blend. However, I've had people tell me my HX620 is not good enough to run my build, which is why I bought the TX750W. True, I plan on eventually getting a GTX 275 or 285, but I don't plan on running sli anytime in the near future.

Is there another component of picking a power supply I'm not aware of? I know the rails are important for GPU draw, but why are people getting 1000+w power supplies if they are only using a single cpu / dual gpu setup? Can someone shed some light on the subject for me?

Thanks.

 

[keenan]

Well it might not be that accurate due to power factor etc., but it's not going to be out by *that* much. Further, because PSUs are only about 80% efficient, the number you see in your UPS is going to be inflated; PSUs are rated for load not their draw from the wall, so you can subtract 20% from that number.

People buy ridiculously large PSUs because they've been successfully marketed; PSU capacity has been creeping up and up for the past years, while power consumption is generally on a downward trend.

Almost any build can be powered by a quality 500w PSU, bar tri-SLI setups. Most single CPU/GPU builds should be fine on a 350w PSU. Anything over 750w is basically nuts.

 

[xazraelx]

Well it might not be that accurate due to power factor etc., but it's not going to be out by *that* much. Further, because PSUs are only about 80% efficient, the number you see in your UPS is going to be inflated; PSUs are rated for load not their draw from the wall, so you can subtract 20% from that number.

People buy ridiculously large PSUs because they've been successfully marketed; PSU capacity has been creeping up and up for the past years, while power consumption is generally on a downward trend.

Almost any build can be powered by a quality 500w PSU, bar tri-SLI setups. Most single CPU/GPU builds should be fine on a 350w PSU. Anything over 750w is basically nuts.

Thank you for the feedback. I'm going to be selling my TX750 to someone because I went from a modular power supply to a non-modular power supply, and well - let's just say the cables are horrendous.

 

[Dangman]

Yes and a quick search would turn up this topic a million times over. Here is the recap:

1) APFC can fool Kill-A-Watts into giving you abnormally low readings (some times giving better than 100% efficiency)

2) Power supplies derate with temperature anywhere from 2w/c above a nominal rated at value to 10w/c.

3) Kill-A-Watt's and most power meters sample too slowly to catch transient loads (the Transient load from our tests is 117w and is COMPLETELY missed by Kill-A-Watts).

4) Power supplies last longer if you stay in the 40% to 60% range of their output.

5) power supplies are quieter if you stay in the 40% to 60% range of their output.

6) Power supplies are cooler if you stay in the 40% to 60% range of their output.

The power meters in UPS software are just as bad. You have to spend some change before you get anywhere near an accurate power meter when your PSU has APFC.

Here's Paul Johnson's post about the inaccuracy of the Kill-A-Watt (Still applies to your UPS):
http://hardforum.com/showpost.php?p=1032190998&postcount=7

The main reason people buy larger PSUs is due to reasons 4-6 above.

In addition, you were only loading up the CPU. Try loading up your GPU and hard drives at the same time. Most people plan for a worst case scenario when it comes to PSUs. Also whoever told you the Corsair 620HX was not enough was dead wrong. According to Corsair's own PSU finder, for a Core i7 920 OC'd more than 30%, an 8800GT and 12 hard drives, the Corsair 520HX would have been enough.

 

[BillParrish]

My thought/question would be, does whatever you ran also put the video card into 3D mode so it becomes a factor in current draw ? If not, the numbers you got would not represent your system in a gaming mode.

 

[Zero82z]

UPS meters can be accurate and they can be inaccurate. There's no way to know if your readings are correct without cross-checking them against a good power meter. Here's an example of a device similar to a UPS power meter reporting inaccurate power draw: http://www.hardforum.com/showpost.php?p=1032190998&postcount=7

However, I've had people tell me my HX620 is not good enough to run my build, which is why I bought the TX750W. True, I plan on eventually getting a GTX 275 or 285, but I don't plan on running sli anytime in the near future.

The HX620 is enough for your system.

Is there another component of picking a power supply I'm not aware of? I know the rails are important for GPU draw, but why are people getting 1000+w power supplies if they are only using a single cpu / dual gpu setup? Can someone shed some light on the subject for me?

The number of rails is insignificant for probably over 95% of people. That isn't something you need to worry about. As for why people get very big PSUs, there are a few reasons. One is that they simply don't know any better so they overbuy because they think it's better. Some people plan on going to a multi-GPU setup in the future so they get a large power supply to have headroom for that. Some just want to be able to keep their PSUs for a very long time without having to worry about not having enough power. The fact is that most typical systems will stay within the peak efficiency range (40-60% load) of a 400W or 500W PSU when running at full tilt.

My thought/question would be, does whatever you ran also put the video card into 3D mode so it becomes a factor in current draw ? If not, the numbers you got would not represent your system in a gaming mode.

He ran Prime95, so no. And even if the GPU was running at 3D clocks, it still has to be under some actual load for it to consume its full power requirements.