What's the rule of thumb for selecting wattage of a power supply?

[3DChipset]

I'm curious, when you see the 80% efficiency on power supplies are they referring to running the power supply at 80% of it's capacity that it's efficiency will be tolerable? For instance if you have a 750 Watt power supply, you would want to run it no more than 80% of it's rating? So everything is running 600 watts in your system a 750 watt supply is sufficient? Anything higher you would want to run 800 watt or higher?

Basically I'm trying to gardner the tolerable limit of running a power supply 24x7....

 

[Redbeard]

No, 80% efficiency refers to the minimum efficiency level of the PSU between 20% and 100% of its maximum load.

For a 750W PSU, that means that anywhere between 150W and 750W, it will be at least 80% efficient.

And by "efficient" I am referring to the amount of power lost during the conversion from AC (wall outlet) to DC (component input) power. Your video cards, CPU, etc, all run off DC power, and converting from higher voltage input (wall is usually 115V AC in the US or 230V AC in EU for example) to lower voltage output (12V for GPUs/CPUs, 5V and 3.3V for some other devices) you lose power. Higher efficiency power supplies lose less power during this process.

The efficiency rating of a PSU just tells you how efficient that is. 80% is pretty standard now, some high-end PSUs (like Corsair AX series) get to a bit over 90% at peak.

 

[3DChipset]

Interesting... Good to know those things. I'm curious though, what is a tolerable level of wattage a 750 watt supply should run 24x7 before it could burn up or run extremely hot? For instance having a load of let's say 700 watts continous on a 750 watt wouldn't be good, right? Is their a rule of thumb to follow by for choosing which PSU would be sufficient instead of just going out and buying a 1000 watt or 1200 watt power supply?

 

[Dangman]

Interesting... Good to know those things. I'm curious though, what is a tolerable level of wattage a 750 watt supply should run 24x7 before it could burn up or run extremely hot? For instance having a load of let's say 700 watts continous on a 750 watt wouldn't be good, right?[ Is their a rule of thumb to follow by for choosing which PSU would be sufficient instead of just going out and buying a 1000 watt or 1200 watt power supply?

This is relevant:

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.

 

[3DChipset]

Thanks for the information Danny Bui .... I'll start taking the 60% rule of the rated power supply. I appreciate the insight.

 

[Yoda4561]

Do your best to figure out the real world power draw of your system at normal/max load. Alot of the online PSU calculators seem to drastically overstate the wattage requirements, so don't rely on those too much. Ideally your PSU will be loaded around 40-50 % at idle and normal use and has a max wattage that will handle your system under the worst case scenario. When in doubt, upsize your psu, the 80+ Gold psu's are efficient enough that you're not adding much to your power bill by operating them at lower wattages.