How much electricity a power supply use?

[zxczxc]

If i use a 400w power supply, is it using 400w all the time or only watts that my pc needs? is it like a 400w bulb? i want reduce my electricity bill... I think 300w PSU is enough for an athlon xp 1700+ system?

 

[Jeff7181]

It's not sucking 400 watts constantly... it's capable of 400 watts total, on all circuits combined.

A quality 300 watt power supply is adequate for an XP1700. But there's other factors to consider too... like everything else in the computer... hard drives, CD drives, video card, fans, etc.

 

[JSSheridan]

When you say that the PSU is 400w, then you are refering to the output power rating. That doesn't tell you how much it draws from the electric utility. The Antec True380 Spec page for example states that it draws 8A @120V or 5A at 240V. That calculates to 960W or 1200W respectively (power = voltage * current). Then the efficiency of the supply is useful / cost, in this case 380W / 960W (1200W) which gives you 39.5% (31.7%.) That is about what I usually see from a supply like this.

If you want to see how much juice you will need, you would need to find out what the maximum consumption of each device is, add them up, and give yourself a bit of head room, and to find out how much power you are drawing from the utility, divide that by your efficiency (decimal not percent).

The amount of power your unit will use will depend on the devices in the system and the efficiency of the supply, but not necessarily the output rating of the supply. You'd actually be surprised by what a 300W supply can power. Peace.

 

[zephyrprime]

states that it draws 8A @120V or 5A at 240V. That calculates to 960W or 1200W respectively (power = voltage * current). Then the efficiency of the supply is useful / cost, in this case 380W / 960W (1200W) which gives you 39.5% (31.7%.)

It's not that simple. You calculations would be correct if power lines were DC but of course power lines are AC. power = voltage * current but both voltage and current are varying. So what you have to do is do a time integral of current draw over the course of 1 sine or cosine AC cycle. And power draw varies over this time period because of ohms's law. Plus it's even more difficult than what I've said because I don't think a PS reacts nice and linearly (after all, it's a switching power supply).

 

[ivan2]

Originally posted by: zephyrprime

states that it draws 8A @120V or 5A at 240V. That calculates to 960W or 1200W respectively (power = voltage * current). Then the efficiency of the supply is useful / cost, in this case 380W / 960W (1200W) which gives you 39.5% (31.7%.)

It's not that simple. You calculations would be correct if power lines were DC but of course power lines are AC. power = voltage * current but both voltage and current are varying. So what you have to do is do a time integral of current draw over the course of 1 sine or cosine AC cycle. And power draw varies over this time period because of ohms's law. Plus it's even more difficult than what I've said because I don't think a PS reacts nice and linearly (after all, it's a switching power supply).

can you give us an approximation of the lower bound please?

 

[JSSheridan]

I know, I know. That was just an approximation written for a Layman. 120V and 8A are RMS values, so calculating the integral is already in the numbers and unnecessary. A PSU would also have a lagging power factor. After another look at the spec page, I see that the efficiency >=68%, a very good rating, possibly optimistic. So that means that the actual watts drawn from the utility would be 380W / .68 = 559W at most. So the PF would be 559W / 960 VA = 0.58 lag which isn't that good, but it won't bring the utility to your house to complain. Also the utility will just bill your for the 559W and not the 780 VAR you also draw (use a^2 + b^2 = c^2) because it doesn't exist, it's imaginary power. I'm also assuming that there isn't any PF correction in the PSU, which I'm sure isn't the case, but it would be very complicated then. That is a pretty complete answer. Peace.

 

[boi]

I bought an electricity meter that plugs into the wall outlet and I measured my computer and it uses about 850watts of electricity under load. I am using an Enermax 431w power supply. It all depends on what type of components you have in your computer.

P4 3.0GHz
1 7200RPM HD
2 CD Drives
Radeon 9700 Pro
Audigy1 Sound Card

 

[Spikesoldier]

look at your little sticker on the PSU, it should say it draws X amps on teh 120V line, which is coming from you wall. use the above forumulas to figure out how many KwH you are using.

 

[Markfw]

This is one of the most intelligent threads I have seen in a while here (I used to be a teacher)

Way to go guys !

 

[JSSheridan]

Markfw9000, you ought to pay a visit to highly technical sometime. That stuff can be very in dept at times. Peace.

 

[0roo0roo]

a quality 300-350 is def good enough for a 1700+

think about it. how do people with dual processors +15k rpm scsi raid harddrives arrays even survive?? 2000watt power supplies? lol

hardware forum/websites peeps get u paranoid about power

look in dells or compaqs of simliar setup to yours, their power supplies are sometimes dinky reallly.

it all comes down to how much is money worth to you. if you have awesome finances, sure go buy a 450watt power supply with blue led fans, just cuz its kewl.

 

[draggoon01]

Originally posted by: boi
I bought an electricity meter that plugs into the wall outlet and I measured my computer and it uses about 850watts of electricity under load. I am using an Enermax 431w power supply. It all depends on what type of components you have in your computer.

P4 3.0GHz
1 7200RPM HD
2 CD Drives
Radeon 9700 Pro
Audigy1 Sound Card

can you measure power consumption when idle or browsing internet? and any other computer systems/laptops at your disposal?

 

[BmXStuD]

a powesupply turns the power into a simple smaller wattage. like your pc is taking the same amount of power that your tv is. So its taking the same as a tv or vcr. Yes i read this in "Computer For Dummies Book".

 

[Zepper]

Actually most switching PSUs are 60-70% efficient. Take 65% for an average. So if your PSU is rated at 300W, then at max load, it should be drawing about 450W. And switching PSUs can adjust to demand, so they don't always draw full power like linear PSUs do.
. It can look different if you don't have a PFC (power factor correcting - syncs the current draw with the voltage draw) PSU - like the one where boi measured 850W draw from a 4xxW PSU. If the difference, on average, is really as great as boi found, then it's definitely worth it to go for an active PFC PSU!!!
.bh.
. Oh, and BTW. The numbers on the label for AC ratings are the maximum possible ratings. Most PSUs will NEVER reach those levels of current draw.bh.
:beer: time!