PSU power input/output confusion

[papercut2008uk]

i'll try to keep it simple, because every time i answer one of these kinds of questions it seems i'm wrong, but i'm pretty sure i'm correct.

 

if you have a 500w 80+ PSU, does that mean the maximum input of power into the PSU is 500w? but the usable power is around 80% so you have in effect around 400w of usable power?

 

or is it, 500w of usable power, so the PSU draws in 600w so it can produce 500w of usable power?

 

i'm pretty sure that it's the first one, but i keep seeing the second explenation being accepted as the correct one.

[Circaflex]

The basic 80 Plus rating means that the PSU is rated for at least 80% efficiency at 20% load, 50% load, and 100% load. The amount of power drawn is determined by the components of the computer and how much they require. The amount of power drawn will only be equal to what is needed, and no more. If all the components of a computer require 300w to run, then the "load" on the PSU will be 300w and hence the power draw of the PC will always be 300w+Inefficiency no matter if it is a 500w PSU or a 1000w. A 500w will not draw more than 500w (in most cases), but it would be no where near 600w.

 

 

Quote

The basic 80 Plus rating means that the PSU is rated for at least 80% efficiency at 20% load, 50% load, and 100% load.

The 80 Plus Bronze rating means that the PSU is rated for at least 82% efficiency at 20% load, 85% at 50% load, and 82% at 100% load.

The 80 Plus Silver rating means that the PSU is rated for at least 85% efficiency at 20% load, 88% at 50% load, and 85% at 100% load.

The 80 Plus Gold rating meas that the PSU is rated for at least 87% efficiency at 20% load, 90% at 50% load, and 87% at 100% load.

The 80 Plus Platinum rating means that the PSU is rated for at least 90% efficiency at 20% load, 92% at 50% load, and 89% at 100% load.

 

[papercut2008uk]

this is why i'm always confused, i'm sure i was right but this is what someone put and they seem to think it's correct.

 

'The 550W is the PSU output to the system, not input wattage. Being 80% efficient, means it will pull 550W/0.8 = 687W at full 550W output to the system.
However, that also means, if your system only needs 400W, the PSU will only pull 500W from the wall outlet. (500W from wall x 80% efficiency, converting it to +3.3V, 5V and 12V). '

 

https://uk.answers.yahoo.com/question/index?qid=20160609183926AA1aVw4

[ShadeOfBlue]

21 hours ago, papercut2008uk said:

this is why i'm always confused, i'm sure i was right but this is what someone put and they seem to think it's correct.

There's no conflict between +Circaflex's answer and that one. They are saying the same thing. The wattage of the PSU is the maximum power it can output. The input power will always be higher because no PSU is 100% efficient.

 

But it's actually (sometimes) more complicated than that. You may have seen how a UPS (battery backup) is typically rated in "VA" instead of "Watts". That's because Watts refers to the power actually consumed, but "VA" refers to the amount of power drawn over the wire. If a device has a lot of capacitance or inductance (like a motor.. or a power supply), then it may move some extra power in and out through the power cord with every cycle of the mains voltage (typically 50-60Hz, depending on the country).

 

However, if the device has what's called Power Factor Correction (PFC) technology, then almost no "extra" power will flow back and forth. If your PSU can accept any voltage (115V or 230V) without flipping a switch on the back, then it does have PFC. But if it doesn't, and the power factor rating isn't listed somewhere.... well usually the industry rule of thumb is to assume a power factor of 0.6. That means you have to take that 500W PSU, divide by 0.8 (625W) to account for inefficiency, and then divide again by 0.6 (1042VA) to account for the power factor.

 

Most regular consumers are only charged for the Watts they use by the power company and not the amount of VA. That extra power isn't actually consumed (it just moves back and forth), but it does need to be considered when deciding how thick power cords need to be or how many VA your battery backup needs to be, etc.