Understanding the PSU

Lotus100

Distinguished
I will like to start this thread but have it continue as a discussion of Power Supplies. Let's see how this goes.

Initial question:
It seems to me by my research that PSU's are rated by wattage factors ranging from 1275W to 500W. Obviously, the higher the wattage the more electricity is drawn. If the computer user uses their computer build say three or four hours each day then how does that translate into your energy charges? Is there a way to predetermine how many kilowatt hours you'll be using by this type of computer use with regard to your choice of PSU?

Solution

http://networking.ringofsaturn.com/Tools/serverpowerusage.php
See if this helps.

Chaz21

Honorable

http://networking.ringofsaturn.com/Tools/serverpowerusage.php
See if this helps.

delluser1

Splendid

The electricity that is drawn will depend on
A) System power consumption
B) PSU efficiency

Get a Killawatt meter
http://www.amazon.com/P3-International-P4400-Electricity-Monitor/dp/B00009MDBU

popatim

Titan
Moderator
PSU typically draw whats needed to run the computer plus a little extra for their inefficiency. It does not matter how much power it could make, only how much you actually uses so ignore the energy calculator posted above.

Efficiency (E) is power output/power input (O/I) times 100 to get percent.
so if E=O/I*100 then to calculate the input power the formulae becomes I=O/E*100

For example 1250w PSU at 80% effeciency running a computer that draws 400w will draw 400w + 100w (400w/80%) = 500w total from the wall. Ie - 100w is being used to create the 400w being used by the computer.

A 650w unit that is also 80% efficient will draw the same amount from the wall: 500W.

A 650w PSU that is 70% efficient will draw 400w + (400w/70%)=571w from the wall costing you 71w more per hour.

Now you get billed by the electric company by the killowatt-hr. A killowatt is 1000w.
It would take the 70% PSU (1000w-hr /71w=) 14hrs of operation to use about 12 cents more than the 80% efficient PSU (depending on the electric rate you pay)

Now this is a simplified example to give you the basic understanding. In reality PSU's do not have the same efficiency across all loads applied to it. Even some of the best 80%+ PSU can have spots where they drop below 80% and other spots where they excell well above 80%.

We also did not touch on apparent power, real power, power factor correction... as these only have little impact on the power being used from the wall.

edited to fix the math. Ty ko88.

ko888

Titan
For example 1250w PSU at 80% effeciency running a computer that draws 400w will draw 400w + 80w (20% inefficient * 400w) = 480w total from the wall. Ie - 80w is being used to create the 400w being used by the computer.
The math seems a little off.

(400W/480W) * 100 = 83.3% conversion efficiency

To get 80% conversion efficiency with a 400W DC power draw would translate to 500W AC assuming a power factor of 1.0.

Lotus100

Distinguished
Chaz21: Wow, super cool answer! I'll have to spend more time with this in the near future. But I think this is exactly what I was talking about in my question. If this does what I think it does then I'll be able to figure out the cost of running a PSU even before I purchase it. Thanks for the heads up!

Lotus100

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delluser1: This confirms my original thinking. I wasn't the only person who wondered about how to figure out the costs associated with my use of electrical power. Wow, this info rocks! Thank you for sharing.

Lotus100

Distinguished
popatim: I am trying to get my head around what you're saying here. I've got to come back and spend some time thinking about what you've pointed out here. Some of what you are saying is easy to follow. While other extensions of your examples are not. I need to come back to your thoughts and think about it more. Thank you anyways for your input.

popatim

Titan
Moderator

You're absolutely correct.
PSU efficiency is =outpu/input*100
It didnt even dawn on me to rework the formula correctly so that I would be dividing by efficiency rather than multiplying by inefficiency.

So yes the 400w output divided by 80% efficiency is 500w input or what is drawn from the wall outlet.

Thank you for catching that.

Lotus100

Distinguished
popatim:

Why are you dividing and multiplying by 100?

Isn't it much easier to just multiple outright? For example, if I have a 1000W PSU and it operates at say an 80% efficiency then isn't it just a matter of multiplication: 1000W * 80% = 800W ? In other words, in this example, I'll be getting only 800W at max efficiency?

Rugger

Splendid
PSUs are rated to provide a certain power...the inefficiency in converting AC to DC means that the PSU draws more power from the wall than it provides to the PC and that is why the calculation is done like that.

popatim

Titan
Moderator

PSU's are rated by their output
we multiply by 100 to get the result into percent form but if you understand that 80% is the same as 0.80 then we are good.

Efficency is output divided by input. E=O/I

Knowing the output and the efficiency, How do you calculate the input? = Algebra time.

1: multiply both sides by I { IE=IO/I }
2: the I's on the right cancel out leaving { IE=O }
3: Divide boith sides by E { IE/E = O/E }
4. The E's on the left side cancel out leaving { I=O/E }

So your 1000w PSU example assuming 80% efficient. I= 1000w/0.80 = 1250W Input (which is what the PSU is drawing from the wall)

Lets check our results.
E=O/I = 1000w/1250w = 0.80

0.80 * 100 (to put it into percent form) = 80%

Lotus100

Distinguished
I understand alegbra my friend. However, I am new to efficiency and how to understand it in terms of what a PSU is inputing from an energy source and outputting to a computer. When I have time next week, I will be spending a bit more time on the learning of this concept. Standby.

Lotus100

Distinguished
I may not completely follow what your math is doing 100% because a) I am unfamiliar with the formula for efficiency, and b) I am also unfamiliar with thinking in terms of kilowatts. However, I do understand alegebra although do not use it frequently. What I do get from your answer is a valuable piece of info: "It does not matter how much power it could make, only how much you actually use...." That makes sense to me!

I also vaguely understand that if a 1250w PSU is 80% efficient
then this must also mean that if my computer is only drawing 400w from the 1250w capacity
then the 400w has also got to be only 80% efficient too. This also makes sense.
The formula for efficiency is what I never knew of before my inquiry.
However, accepting what you've put forth as fact, yet still being confused about this due to my unfamiliararity,
the 400w/80% = 500w
And the 500w is what I need to put into the calculator suggested by a prior user [see name above].
This begs the question: How do I determine how much power my computer will actually be using. And for this perhaps the above calculator may prove helpful.

I appreciate your input and thank you for your contribution to my learning experience. I am still making time to study this further. But I did want to make a comment to you about your response here (now editted).

I don't know if I'm allowed to pick multiple best answers, but I have given credit to the first three responses.

My time is limited on further study of this topic, but I will be reviewing this again in the near future. Thanks again for shedding some light on this subject.

Lotus100

Distinguished
popatim

Now, I think I'm getting a clearer understanding of things.
Using a 1250w PSU at an 80% efficiency
Having a computer drawing 400w
This means 400w/80% or 500w
That's 100w greater than what the computer actually needs to operate.

Your writings are unclear on exactly how many watts I am paying for. In one response above you seem to indicate that it takes 100w to generate 500w using a 1250w PSU at an 80% efficiency rate.

Now am I paying for just 100w in the above example?
Or am I paying for the entire 500w in the above example?

That's where you are confusing me. But I think I get what you are saying if the latter question is what I am paying for in wattage (in the above example).

Please confirm my understanding or clarify. Thanks.

ko888

Titan

In your above example it takes 500 Watts AC to generate the 400 Watts DC. There is 100 Watts being lost in the AC-to-DC conversion process. That 100 Watt loss shows up as heat that the power supply gives off.

You are paying for the 500 Watts you are drawing from the AC wall outlet.

Lotus100

Distinguished
ko888: Thank you. Now I do believe I have my head around this rather complicated subject. And I know how to use the wattage calculators described in other posts under this topic. I am very pleased to see there are people like you that are offerring to clear things up for people like me who are learning about things. Go buy yourself a soda or coffee, and have a nice day.