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My desktop computer is an i3-2100 with 1 DVD drive, 2 HDDs and 2 SSDs connected to it, as well as a CPU fan and a case fan. All of this is connected to my PC via extension cord (i.e. power bank), which is in turn connected to a plug-in energy monitor at the mains socket.

When my last PSU died a few years ago, I bought my current one after some calculations to determine the wattage of all of the components, and those calculations gave me roughly 300W, with less powerful components than I currently own. I ended up buying a 550W Corsair PSU just to be safe. However, my energy monitor shows the output of all the above components as being between 80-90 watts, or 3.5 kWh.

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This seems incredibly low to me, especially given that simple household appliances like hairdryers can use up to 1000W. Is this figure likely to be accurate, or is my energy monitor broken?

Hashim Aziz
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  • A computer idling uses less energy. – DrMoishe Pippik Jan 03 '19 at 00:35
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    [This review](https://bit-tech.net/reviews/tech/cpus/intel-core-i3-2100-review/7/) measured the power consumption of a machine with the same CPU as yours and got 78W to 115W depending on what the machine was doing. I'm not sure what you think would take so much power. 70W is max for the CPU. 5W per drive, maybe 2W per fan, 10W for miscellaneous stuff. – David Schwartz Jan 03 '19 at 00:57

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The meter is probably OK and the wattage figure is likely fairly accurate at the time it was taken - and the computer was most likely not doing much at that point in time.

The figure of 3.5kWh is meaningless without context - ie over what time the measurement is taken but seems unlikely or does not equate with your 80-90 watts. Assuming your system uses 85 watts, that means a draw of 85watts per hour or 2.04kWh per 24 day. Of-course, if you were measuring usage over an extended period, then stopped and looked at your kWh reading and wattage, this makes sense as the computer was using a lot more power (probably 200 watts or so) when active, but very little when you were not working on it and looking at the meter.

There are a few things to take into account -

  1. When a computer is idle it uses a fraction of the power that it would use when under load. (Think gaming maxing out CPU and GPU)
  2. A PSU needs to be able to handle start-up/inrush currents, when a device is turned on. Very often this sudden surge of power is very high compared to operating current, but still needs to be handled by the PSU.
  3. It is unclear what components you compared your system to, however more powerful does not neccessarily mean more power is consumed - particularly in the case of newer generations of hardware compared with older ones, or things like motherboards and disks.

If you think about it, a typical i3 laptop uses less then 45 watts, so 80 watts is entirely reasonable for a system which is not doing heavy GPU and CPU calculations. Your CPU is designed around 65watts UNDER LOAD, and this would likely be the biggest consumer of electricity. When not under load, they use very, very little power.

From another POV - For practical purposes, the amount of energy a computer uses is roughly equivalent to the amount of heat it dissipates. Which is putting out more heat - your hairdryers or computer?

davidgo
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  • FWIW, the SSD's probably use less then 5 watts each when used, the hard drives less then 10 watts each when spinning. A DVD drive would use about 25 watts, but only when actively working. – davidgo Jan 03 '19 at 00:55
  • That answers my questions and then some. I especially didn't know about inrush currents, and knowing about them makes the need for the higher wattage PSUs on the market seem more logical. I'm not sure what's going on with the KWH thing, other than this is what I see when I switch the energy monitor from *Watts* to *KWH*: https://imgur.com/yl04M4f. – Hashim Aziz Jan 03 '19 at 00:59
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    The kWh thing measures your usage over time, so if your system uses a lot of power and then very little, it will average your usage so it can produce a meaningful number that you can multiply by what you pay per unit of electricity so you know what it has cost you. My guess is its the amount of electricity you have used since you plugged the PC into the meter - but unless you know how long a period it is measured/calculated over, the figure does not mean anything. – davidgo Jan 03 '19 at 01:07