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Please help me understand power efficency

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August 31, 2007 7:08:49 AM

Hi, I failed electronics in high school, so I need help understanding math.

My pc has

  • 2.4ghz Athlon x2
  • 7800gt
  • 4 Hard drives
  • 2 Disc drives
  • 1.5gb ram

    I would like to know if at 6.2 cents per kwh,this system will cost much to run over time. I'm asking because I'm moving into a new place, where the landlord pays the hydro and I'm concerned that I will cause a large spike in the bill. I'd appreciate someone's help with this.
    a b B Homebuilt system
    August 31, 2007 2:50:02 PM

    jijeewu said:

    I would like to know if at 6.2 cents per kwh,this system will cost much to run over time. I'm asking because I'm moving into a new place, where the landlord pays the hydro and I'm concerned that I will cause a large spike in the bill. I'd appreciate someone's help with this.


    What is your question? exactly? I don't get it!
    August 31, 2007 10:17:38 PM

    itll probably make as much difference as having 2 or 3 normal 100W light bulbs on. Unless you use your pc constantly it really isnt too important, uses less than boiling the kettle.

    anyway for the record

    300W pc (average ish as it varies) X hours spent on it (4 a day)

    300 X 4 = 1.2 KWH so thats like 10C a day or $3 a month

    obviously i'm just guessing on the power usage but it shouldnt be too much more
    Related resources
    a b B Homebuilt system
    September 1, 2007 5:26:56 AM

    1. If you still can remember, there are 2 kinds of power.
    kW und kVA.
    kW = kVA / cos P
    Cos P (Power factor) ranged from 0.0000000001 to 0.99999999999
    As you can see the higher the power factor is, the more likely kW = kVA.
    kW is the ammount of power that your components used and kVA is the power given/obsorbed.
    You pay the kVA, not the kW.
    The value of the power factor is determined by the components itself.
    Good quality components have better power factor.
    In some countries, a company (not home usage) will receive an extra punishment bill if they used less than 0.90 of power factor.
    So the lower the power factor the more you pay.
    (I tried to put it simple, because it is not that simple. If you like to know the details, just hit the books.)

    2. The unefficiency caused by heat dissipation and more. (I don' want to explain this)
    September 1, 2007 6:07:52 AM

    guanyu210379 said:
    1. If you still can remember, there are 2 kinds of power.
    kW und kVA.
    kW = kVA / cos P
    Cos P (Power factor) ranged from 0.0000000001 to 0.99999999999
    As you can see the higher the power factor is, the more likely kW = kVA.
    kW is the ammount of power that your components used and kVA is the power given/obsorbed.
    You pay the kVA, not the kW.
    The value of the power factor is determined by the components itself.
    Good quality components have better power factor.
    In some countries, a company (not home usage) will receive an extra punishment bill if they used less than 0.90 of power factor.
    So the lower the power factor the more you pay.
    (I tried to put it simple, because it is not that simple. If you like to know the details, just hit the books.)

    2. The unefficiency caused by heat dissipation and more. (I don' want to explain this)


    From your power meter's point of view, KVA and KW are the same. KVA is just a more universal way of expressing KW, as V will vary country to country, thus A as well. Still goes back to Ohm's law, P=IxE. As an example, my UPS draws 1.2 A from the wall with everything connected to it operating at a medium to full load. That's a PC with 2 video cards, three HD's, 2 DVD RW... a 24" LCD, a NAS box, several phone chargers, a wireless router and my modem. In AUS we get a nominal 240 V supply, so I am using 1.2 A x 240 V, or 288 W. (In the US I'd probably use more like 2.4 A, but the watts would remain the same.) So my fairly high end system IN TOTAL uses .288 KWH. In your case that would equate to... rounded up, 1.8 cents per hour.

    Now as for the 'power effeciencies' bits, your PSU is taking that AC and converting it to DC for use inside your PC. This is not a loss-less conversion, so if your PSU runs at an effeciency of 80% it will draw more watts from the wall to run the same system than a PSU that is 90% effecient.

    This is a rather simplified explanation, but hopefully one that the OP can understand.

    September 1, 2007 6:39:35 AM

    your rig probably consumes, at max during heavy computing/game, about 150-175 watts, at the outlet, meaning about 6 hours of continuous gaming would cost you 6.5 cents in energy. Idle power consumption I would estimate to be around 80 watts (those 4 hard drives take up a significant amount of idle power consumption, probably around 20 watts total).

    The display type and size was not mentioned. If you're using a CRT monitor, add about 100 more watts to your power consumption. If it's a flat panel, power consumption will vary from 10 watts for a little 15" to 50 watts for a 20" monster.

    If you want to save on energy, the best thing you can do is to switch to CFL lightbulbs. That's way more electric savings than your computer will ever use. Although there are a few decent power saving computer tips: instead of screen saver have the monitor go into standby, don't run programs like F@H or SETI, and turn off the computer when you're not using it. Those 3 things might save you a few bucks every year.
    September 1, 2007 6:45:44 AM

    guanyu210379 said:
    1. If you still can remember, there are 2 kinds of power.
    kW und kVA.
    kW = kVA / cos P
    Cos P (Power factor) ranged from 0.0000000001 to 0.99999999999
    As you can see the higher the power factor is, the more likely kW = kVA.
    kW is the ammount of power that your components used and kVA is the power given/obsorbed.
    You pay the kVA, not the kW.
    The value of the power factor is determined by the components itself.
    Good quality components have better power factor.
    In some countries, a company (not home usage) will receive an extra punishment bill if they used less than 0.90 of power factor.
    So the lower the power factor the more you pay.
    (I tried to put it simple, because it is not that simple. If you like to know the details, just hit the books.)

    2. The unefficiency caused by heat dissipation and more. (I don' want to explain this)


    Wait a tick, you Euros have to pay for apparent power? That sucks. All my previous power estimates were for actual power used (including PSU inefficiency), not apparent power.
    a b B Homebuilt system
    September 1, 2007 6:41:53 PM

    croc said:
    From your power meter's point of view, KVA and KW are the same. KVA is just a more universal way of expressing KW, as V will vary country to country, thus A as well. Still goes back to Ohm's law, P=IxE.


    Yeah right! Do you understand what cos phi or power factor means? really? do you really2 sure 'bout this?
    Why don't you do a little searching 'bout that in the books!
    Do you understand bout how coils and capacitors can effected your power consumtion, harmonic distortion, how to deal with under value of power factor and etc. ?

    Get real dude. I don't be that sure, if i were you!
    Altough this time, yup ! , for home users kVA and kW didn't make much differences.
    And I've made a little missprint!
    It should be :
    kVA = kW / cos P
    Your computer consumed 600W (let asume you have a very very very very very good PSU and the power factor is 0.8)
    kVA = 0.6 / 0.8 = 0.75kVA and you did have under 0.9 power factor value. In most countries, if you are a company than you have to pay an extra fee because of it but since you are a home user you don't have to pay it. This fee is not the same for every countries.

    Let see what you've wasted as a company and consumed 600,000 kW instead of 600W.
    kVA = 600,000 kW / 0.8 = 750,000 kVA . You'll have to pay this bill + extra fee + your power network are burdened by loses caused by low on power factor.
    kVA = 600,000 kW / 0.95 = 631,579 kVA . You'll just have to pay this bill and without extra fee + your power network are not burdened by loses caused by low on power factor.

    The price for the bill you've paid every months is with the calculation of kW and that goes the same to the companies, but the big big difference is in this extra fee. And this fee also came together with a warning, if you didn't fix the power factor, you are dealing with the law! not to mention the loses caused by low power factor.
    You don't pay the kVA but the kW. That why i have said ''Altough this time, yup ! , for home users kVA and kW didn't make much differences.'' because you don't have to pay the extra fee. An,d you use only a little amount of power, unlike the companies.

    Why or where did this power factor come from? For further details just hit the books man or ask any power electrical professors at your nearest university ! It is a very long story! And I'm not interested to give all the details here!
    I'm trying to make it as simple as i can, because it is not that simple, infact it took years for somebody to learn it correctly in the university. So a lot lot lot lot lot lot of things are missing here, then it is normal and are intended to be made.
    And this P = I x E can only be used if your system are consists with only resistors!
    You should be carefull before you said anything and if that turned out to be wrong!

    September 1, 2007 8:14:12 PM

    jijeewu said:
    I would like to know if at 6.2 cents per kwh,this system will cost much to run over time. I'm asking because I'm moving into a new place, where the landlord pays the hydro and I'm concerned that I will cause a large spike in the bill. I'd appreciate someone's help with this.


    Here's the basics.

    First, though others in the thread have a legitimate technical argument going over the power factor, don't worry about it. It's not gonna make a hill of beans difference in terms of your landlord's overall bill.

    One of the previous posters was in the ballpark on his estimate ... your computer will run $3-$5 a month if it's in the 300-400W range (for everything), and if you use it 4 hours a day.

    Computer costs will begin to really eat into your bill if you leave the machine on all the time. If you instead leave the machine on 24/7, you'll be looking at $20-$30 a month.

    An efficient power supply can help you. Efficiency is basically this: The power coming out of the wall mains is one voltage, the computer needs the power at a different voltage. The power supply's job is to perform this conversion. The conversion process isn't perfect - the power supply wastes some power during the process. The wasted power is given off as heat.

    The more efficient power supplies waste less power for the same amount of conversion. That's it in a nutshell.

    The wasted power costs you twice: First because you have to pay for it in your bill (the power supply converted it to heat instead of running the computer with it), and second because then your air conditioner in the house must remove that heat.

    Efficiency of a power supply is generally given as a percentage. The percentage is the amount of mains power that actually goes to run the computer. The remainder is the power that's wasted as heat.

    Example:

    70% efficient power supply running a 300W computer: Computer uses 300W, which is 70% of what the power supply is taking from the mains: 300/.70 = 428W coming in on the mains. 128W of that is wasted as heat. At 6.2 cents per kWh, and if the computer is run 24 hours per day, 128W * 24 * 0.062 * 30 = $5.71 that you pay per month just for the heat. The A/C in your house has to remove this heat too, so count another few dollars for that.

    90% efficient power supply running the same 300W computer: Computer uses 300W, which is 90% of what the power supply is taking from the mains: 300/.90 = 333W, only 33W wasted as heat. 33W * 24 * 0.062 * 30 = $1.47 for the heat, and now only a few cents for the A/C heat removal. The better power supply has saved you probably $6-$8 per month for a 300W computer running 24/7.
    !