Computer equipment: Current and Power

There's quite a lot of discussion of standby power nowadays. It would be interesting to see how much computers really take when on standby, but it's not obvious that cheap power meters will measure this properly when there may be very capacitive currents from filters, or very non-sinusoidal currents. Also, for those of us who hardly know what standby is, our computers being left on day and night, it would be interesting to know the computers' running consumption, and how much this is affected by CPU activity or number and activity of hard-disks. The currents drawn by most computer equipment are highly non-sinusoidal in any mode of operation; what is the form of the current?

These questions are answered for some computers and monitors, in the following results. Measurements have been made with an oscilloscope (Tektronix TDS 3052), of the voltage across and the current into these devices.

For the interested, there is some more detail about the measurement method.

After doing these measurements, one simple plug-in power meter was found to be quite accurate even with harmonics and capacitive loads, so some more appliance_powers were measured for some domestic loads, including just `power' (W) and `apparent power' (VA) rather than the full harmonic details.

There are also some old measurements of harmonic currents (and general power consumption) of `low energy' lamps.

The supply voltage

The supply voltage was close to its nominal 230 V rms for all the measurements. The waveform's features, mainly a depression just before the peak, were quite steady over time.

Three example waveforms are shown in top of the following figure, besides a pure sinusoid (dashed line) as a reference. Below that, the spectrum of voltage amplitudes at some low harmonic frequencies is shown, for all of the measurement occasions.

The "dc" level (0th harmonic) cannot be taken seriously, as the oscilloscope is known to have some small voltage-offset that was not taken into account back in the days of these quick measurements.

Calculated quantities

For each item, the currents are shown plotted against time, with one curve for each operating mode measured.

The matlab script that was used to process the results is here: plot_it_power.m. The results from the oscilloscope are condensed in this: raw_data_vi.m, called by the above script; just current and voltage columns from the original CSV files have been used, verbatim (the times are always taken as being 0--40 ms).

Some single-value indices have been calculated for the v(t) and i(t) curves.

`backup': AMD64 desktop computer

This is a home-made desktop machine, currently in use as a headless backup server, with three disks. It runs RedHat Enterprise 5, 64-bit. The processor is an AMD Athlon64 3700+, which runs at 1000 or 1800 or 2200 MHz depending on loading (`AMD power-now'). The memory is two lots of 400 MHz DDR, and the motherboard is an Asus A8V-E SE with VIA K8T890 chipset.

Five different states were used. In the order in which the results are given, these were:

    Name                             Vrms   Vpk     Irms   Ipk      Srms   Preal   PF
   'backup_off'                     228.0  328.8   0.054  0.154    12.2    5.6    0.46
   'backup_3spundown'               227.8  330.9   0.363  0.888    82.6   63.6    0.77
   'backup_normal'                  228.3  332.5   0.439  1.007   100.1   79.0    0.79
   'backup_fullcpu'                 227.8  328.8   0.722  1.620   164.6  126.9    0.77
   'backup_fullcpu_muchdisk'        227.2  328.1   0.779  1.742   177.0  136.0    0.77
i(t) for AMD64 desktop computer

Clearly, the CPU loading has a strong effect, increasing the 80 W idle power by 45 W. Heavy use of three disks made only a 10 W increase above this (try again with disk loading without high CPU use?). Turning off three idle disks made about a 15 W reduction of the 80 W idle power. Even when supposedly turned off (at the front) this computer takes about 6 W.

`solar': Pentium4 desktop computer

This is a Dell Optiplex GX260, with a 2.4 GHz P4 (no HT); it contains either 4 disks (all about 40 GB, four to six or so years old) or just 1 disk by disconnecting the other three from the power and signal connectors before booting. The operating system is Solaris 10 x86 (05/08), running with GUI.

Five different states were used. In the order in which the results are given, these were:

    Name                             Vrms   Vpk     Irms   Ipk      Srms   Preal   PF
    'solar_off'                      228.7  332.4   0.121  0.213    27.7    2.1    0.07
    'solar_onguiidle1disk'           228.2  330.3   0.281  0.788    64.2   41.7    0.65
    'solar_onguiidle4disk'           228.6  330.6   0.358  0.972    81.8   55.9    0.68
    'solar_onguiidle1diskthrash'     228.3  331.6   0.304  0.854    69.5   46.8    0.67
    'solar_onguifullcpu4disk'        226.9  328.4   0.587  1.452   133.3   98.7    0.74
i(t) for P4 desktop computer

This suggests that idle hard-disks don't take much power; it is about 14 W change for three hard-disks being present rather than not. The difference between idle and hard-working disks is again small, about 5 W each. The difference between CPU load being zero and full is much bigger, a bit over 40 W, very similar to the AMD64 case although the P4 doesn't have an explicitly mentioned speed reduction for power saving (?). Note that the off-state current (and the others states' currents, less noticeably) has a component suggestive of a [noisy] capacitor current, in contrast to the current in the other computer `backup'.

`p3fn': a single PIII 600MHz, 3-disk fileserver

This is a normal, old desktop computer, adapted to hold three disks connected to a hardware raid controller in raid5, as a fileserver. The OS is FreeNAS (a FreeBSD derivative), running on a flash disk. In this case we also have the opinions of two different makes of power-meter, to compare with the oscilloscope's results. The measurements by the three different instruments were not made at the same time, since the instruments consumed considerable reactive power themselves, so any `daisy-chaining' of the measurements would have loaded up-stream instruments with the down-stream ones.

Four different states were used. In the order in which the results are given, these were:

Turning off the switch on the back of the computer brought the power to zero. It should be noted that although the chassis and CPU/motherboard presumably date from the `turn of the millenium', the power supply unit is an expensive modern one, and the currents can be seen to resemble more those of the low power-factor server `penguin' than the other items on this page.

At the end of the following table, the results of power (`W', suggesting active power) of both of the plug-in power meters are given.

	Name                 Vrms   Vpk     Irms   Ipk      Srms   Preal   PF	   M1  M2
	'p3fn_off'           226.6  332.1   0.245  0.427    55.4   -0.8   -0.01    22   1
	'p3fn_idle'          226.2  331.4   0.412  0.997    93.2   62.2    0.67    69  61
	'p3fn_fullcpu'       226.7  332.0   0.460  1.118   104.3   75.0    0.72    84  76
	'p3fn_full3raid'     229.6  336.9   0.504  1.274   115.8   83.0    0.72    88  83
i(t) for P3-based fileserver

It is noteworthy that the difference between idle and fullcpu is pretty small, less than 25% increase. The hard use of disks gives a bigger increase, but this activity causes about 80% CPU use anyway, so part of the increase is CPU even in the full3raid case. This isn't a very good example of a PIII computer, on account of the unusually new and high-quality power supply.

`penguin': a dual Opteron, 8-disk fileserver

This is a rack-mounted fileserver, 2006 vintage, with dual Opteron 248 2.2 GHz CPUs, 2 GB of memory, and eight SCSI U320 10krpm disks. One disk is for the system, six are in RAID5 (linux 'md raid') and the eighth is a hot spare. The OS is RedHat Enterprise 5, 64 bit. There are three power supplies in the computer's back, each able to run it at full power; this feature was used to make the measurements without turning the server off.

Five different states were used. In the order in which the results are given, these were:

    Name                             Vrms   Vpk     Irms   Ipk      Srms   Preal   PF
    'penguin_idle3pow'               225.7  326.1   0.420  0.895    94.8   82.1    0.87
    'penguin_idle1pow'               225.2  326.3   1.035  1.970   233.2  219.5    0.94
    'penguin_diskload'               225.8  328.6   1.405  2.589   317.2  301.7    0.95
    'penguin_fullcpu'                226.0  326.2   1.440  2.544   325.4  310.2    0.95
    'penguin_fullcpudiskload'        225.2  325.1   1.522  2.640   342.8  327.7    0.96
i(t) for fileserver

This is the only bit of equipment seen here that has its current quite well centred on the voltage peak and spread over much of the cycle. The power factors are quite high, and harmonics beyond the third are very small.

Full CPU loading adds only about 40% to the consumption. Disk usage makes little difference: about 20 W for all. Spinning down disks was not tried, as there is almost always some use of the disks. Since the CPU usage due to the heavy file writing is about 1½--2 fully loaded CPUs (mainly in `system' state rather than user), the comparison of fullcpu and fullcpudiskload is the best estimate of disk consumption. It was not noted whether the apparently highly loaded CPUs during copying were running at low or full speed. During heavy disk use, the current trace on the oscilloscope moved up and down by a factor of some 25% or so; a middlish value was taken for the capture and data transfer.

`crtmon': a CRT 21" 1600x1200 pixel monitor

This is a large, Trinitron-based Cathode Ray Tube (CRT) monitor, Dell model P992. Rated `power' is 2 to 1 A, at 100 to 240 V; there's no mention of peak, rms, etc.

The power consumption, when off at the button on the front, is very small, yet the apparent power is much more; a filter or power-factor capacitor is probably responsible for the quite sinusoidal phase-leading off-state current. The `onblank' state is when the monitor is turned on but with only a dark screen to its signal input. `ongui' is running with a colourful login screen being displayed. The power consumption is quite high -- nearly 100 W.

    Name                             Vrms   Vpk     Irms   Ipk      Srms   Preal   PF
    'big_crtmon_off'                 228.7  332.5   0.072  0.143    16.4    0.6    0.04
    'big_crtmon_onblank'             229.3  331.7   0.412  1.041    94.5   68.2    0.72
    'big_crtmon_ongui'               227.8  328.6   0.541  1.312   123.3   91.8    0.74
i(t) for CRT monitor

`tftmon': a TFT 1280x1024 pixel monitor

This is a flat-panel TFT monitor, Dell model 1707FPf, rated at 2 A, 100 to 240 V. The current suggests a shunt capacitor somewhere on the input, and the input converter clearly takes a pulse centred on the voltage peak, much smaller on standby than on normal operation. Power consumption is only 30 W or so.

    Name                             Vrms   Vpk     Irms   Ipk      Srms   Preal   PF
    'tftmon_off'                     227.3  331.2   0.032  0.081     7.2    0.6    0.08
    'tftmon_ongui'                   228.5  331.1   0.268  1.031    61.3   28.7    0.47
i(t) for TFT monitor}

Page started: 2008-08-02
Last change: 2009-02-13