PC Wattage

Discussion in 'Electricians' Talk' started by Al3, Sep 24, 2021.

  1. Al3

    Al3 New Member


    I'm trying to understand the real wattage of a pc.

    Lets just say it's @ 400W.

    Watts = VA

    If it's 400W @ 12 volts, it's going to be a lot less at 240v right? As the voltage has gone up.

    Is it as simple as 12 is 1/20th of 240 so it will be 20 watts at 240v (plus PSU conversion losses) or is there a different way to calculate this. Does the curernt have a bearing here - will that have changed as well?

    Thanks in advance.
  2. AnotherTopJob

    AnotherTopJob Screwfix Select

    The actual consumption will depend upon the hardware fitted and what tasks are being carried out.
    The power supply might be rated at 12v 400w for instance, but if it's just running a basic motherboard with built-in graphics it could only be consuming 50w.
    A high-end processor and graphics card could easily use the full 400w during intensive gaming but half that during word processing.
    The only real way of getting an accurate figure at mains voltage would probably be to use an energy monitor.
    ElecCEng and Al3 like this.
  3. DaveF

    DaveF Active Member

    No, wattage is the same no matter what the voltage is. So if the PC consumes 400W at 12V it will consume the same 400W at the mains inlet of 240V or whatever. In fact nothing is 100% efficient so it will consume slightly more. A PC that has say a 300W graphics card will actually consume 350W (ish) to drive that graphics card because some of the power is lost converting the electricity from 240V to 12V. So the power is constant, which tells you that when the voltage decreases the current increases.
    An average PC consumes around 100~150W when it's doing nothing. In gaming it will typically use 250W-450W very much depending on what is in the PC and how hard you push it.
    Al3 likes this.
  4. MGW

    MGW Screwfix Select

    In the main watts = volts x amps x power factor correction, but that does not seem to work with sound, never worked out why, but a pair of 300 watt speakers on a 300 watt amplifier don't seem to use even 2 amp at 230 volt, I would assume down to balanced circuits?
    Al3 likes this.
  5. Al3

    Al3 New Member

  6. Jimbo

    Jimbo Screwfix Select

    The power draw to drive loudspeakers is usually really low in a domestic setting, even if you like it loud. 80% of the power is in the first few four octaves and the thud of a bass drum will certainly draw current from the amplifier power supply, supplied from it's smoothing capacitors. But the average power draw would be low. Since one single watt will produce over 90dB on almost any system, not a lot of watts are needed. As a point of reference, my late father-in-law had a mobile disco in the 80s and 90s running from 2x 100W power amps. There were never complaints about being too quiet! Then again PA drivers are often approaching 100dB sensitivity.

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