Does a PSU always draw its full wattage

I'm trying to calculate how much it will cost to leave my PC on 24/7. I'm not sure if I need to use my PSU max rating in the calculation or what it's actually drawing idle though.

Heres my specs:
Asrock Extreme 4 970
12GB RAM (4 sticks total)
Hyper 212 Evo
Antec Truepower Clasic 550W
3x 140mm fans
1x 120mm fan

I feel like this system wouldn't draw a full 550W especially how it idles at 10% CPU usage. So how does this work? Does it always draw full wattage or can I figure out exactly how much we draw idle? My computer will be basically idle when on so that's the number I'd like to use.

7 answers Last reply Best Answer
More about psu draw full wattage
  1. The PSU will only draw what is needed at the current time. :)
    It's hard to calculate exactly how much it'll pull at idle, but it's really not much at all.
  2. No. It draws what is needed at that moment by your computer. Borrow an amp meter, and you can see how much power it's using.
  3. the 550W is the max wattage, so it won't draw a 550W all the time, it will vary on how much load you put on it.

    If that was so, then a typical PSU would only last probably 1-2 years.
  4. No, the PSU only draws a bit more than the system needs.

    Let's say you have a 1000W PSU that is 80% efficient at 20% load. If the system draws 200W of power, it'll put the PSU under 20% load. The power draw from the wall can be calculated like this:
    X=power draw from the wall, 80%=0.80
    X*0.8=200W <=> X=200W/0.8 <=> X=250W.

    Of course it's hard to know how much your system actually consumes while idling, so getting a power meter would be easier:
  5. Best answer
    No. It is "on demand". for idle power consumption go with 100w for your calculations. It will be pretty close, but you could bring that up a bit if you wanted to pad your calculations on the high side...
  6. Thank you everyone for the perfect answers. Very very good news that the PSU takes power as needed :P Best answer for sadams because he gave me an estimation on power usage, I know I'll need to get a meter to find an exact reading but I don't believe I have one I can use at the moment.
  7. NO
    a PSU only takes that much of wattage that its <output> hardware wants
    the best example is the circuit of your house even if the switch is ON but no appliance is connected no electricity is used..........
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