PC Power Consumption: How Many Watts Do We Need?

This investigation will not only discuss the new equipment we are going to use from now on for quick, easy and detailed power consumption analysis of complete computer systems and individual components. We are also going to talk about a few typical system configurations and try to determine what power supply will be sufficient for them.

by Oleg Artamonov
06/07/2009 | 05:25 PM

Choosing a power supply for a specific computer configuration is a kind of an eternal problem, especially when the configuration is too advanced for the typical 300-400W PSU bundled with the system case to cope with. You can take it easy and buy a PSU with a wattage rating of some 1000W, but this is likely to be a waste of money. Unfortunately, there are often no comprehensive data on the power requirements of specific components. Graphics card and CPU makers want to be on the safe side and declare overstated specifications. Various power consumption calculators use obscure numbers. And the measurement of real-life consumption, even though mastered by most computer-related media, is often deficient.


When you open the Power Consumption section in a hardware review, you will usually find power draw data as measured from the wall outlet. That is, how much power is consumed from the 220V (or 110V if outside Europe) mains by the power supply which is loaded by the tested computer. It is easy to perform this kind of measurement: a consumer wattmeter is a small device with a single connector selling for less than $50, which is very cheap in comparison with serious measurement tools.

Such a wattmeter usually has a rather high accuracy, especially at loads of hundreds of watts, and copes with nonlinear loads (any computer PSU, particularly without active PFC, is an example of such load): there is a special microcontroller inside it for integrating the current and voltage over time, allowing to calculate the amount of active power consumed in the load.

Nearly every computer-related test lab has such a device.

We’ve got one, too. However, we use it only when we need to quickly estimate the power consumption of a computer or some other device (a consumer wattmeter is most handy then because it needs no kind of preparation) but not for serious tests.

The fact is, although this measurement method is simple, it provides an impractical result:

The obvious, although technically more difficult, alternative is to measure the current consumed by the load proper from the PSU. It is possible, though, and I have even tested the Gigabyte Odin GT power supply with a built-in wattmeter of that kind.

In fact, the Odin GT can make a complete power-measuring testbed in its own right (and I don’t understand why other media don’t use this PSU for such measurements and why Gigabyte does not use this as a promotional opportunity) but I want to build a testbed that would be more universal and flexible in terms of how the load is connected to it.

Our Testing Equipment and Methodology

The simplest method – to insert current-measuring shunts (low-resistance resistors) into the cables going from the PSU – was discarded immediately because shunts rated for high currents are rather large and have a voltage drop of tens of millivolts, which is quite a lot, for example, for the 3.3V rail.

Good for me, Allegro Microsystems turns out very fine linear current sensors based on the Hall effect: they measure the magnetic field created by the current flowing in the conductor and transform it into the output voltage. These sensors offer a number of benefits:

So, I took Allegro ACS713-30T sensors rated for a current up to 30 amperes.

The output voltage of the sensor is directly proportional to the current flowing through it. So, the desired result can be obtained by measuring that voltage and multiplying it by an appropriate coefficient. The voltage can be measured with a multimeter, but that’s not expedient because it is manual labor. Moreover, typical multimeters do not have a high response. And I would need multiple multimeters in order to measure the currents in the different channels simultaneously.

So I went further and decided to make a complete data collection system by adding a microcontroller and ADC to the current sensors. An 8-bit Atmel ATmega168 was selected as the ADC. Its most important resource is an 8-channel 10-bit analog-to-digital converter that allows to connect up to 8 current sensors to a single microcontroller. And I did connect them:

Besides the microcontroller and eight ACS713 sensors, you can see a (relatively) large FTDI FT232RL chip. It is a USB interface controller via which the measurement results are loaded into a PC.

The system is rather compact measuring about 80x100mm (without the USB connector). It can be mounted right on a PSU and the PSU can be then installed into standard ATX system cases. The photo above shows the card connected to a PC Power & Cooling Turbo-Cool 1KW-SR power supply.

The system must first be calibrated. A known current is driven through each channel, and the correlation between the current and the output voltage of the ACS713 sensors is calculated. The resulting coefficients are stored in the microcontroller’s ROM and are bound to the specific card. The card can be recalibrated whenever necessary and write new coefficients into the ROM.

The card is connected to the computer via USB. And you can even use the same computer whose power consumption you are measuring. There are no limitations here. In some cases you may want to measure from a second computer in order to draw the power consumption graph right from the moment the Power button is pressed.

A special program was written for the card. It can get data in real-time mode and display them in a diagram. The diagram can later be saved as a picture or text file. The program allows to choose a name and color for each of the eight channels and reports minimum, maximum, average (over the entire period of measurement), and instantaneous values. The total of the currents in the same-voltage channels and the overall power consumption are calculated, too. But as the testing tool does not measure the voltages proper, the power consumption is calculated basing on the assumption that they are exactly 12.0V, 5.0V, and 3.3V.

By the way, there is one tricky thing: measuring the peak consumption on each power rail and summing them up for all the rails is not enough because these maximums may have occurred at different moments. For example, the HDD had a consumption of 3A five seconds after the start of the system when spinning its spindle up, but the graphics card had a consumption of 10A after FurMark had been loaded. Does it mean that the total maximum consumption is 13A? No. Therefore the program calculates the instantaneous consumption for each time moment and chooses the maximum out of them.

The measuring card is polled 10 times every second. The polling period can be increased, but this is not necessary for most applications: there are too much data while the result does not change much.

Thus, we have a handy, flexible (the card can be connected to the PSU in different ways depending on the purpose of the particular test), simple to connect and use, sufficiently accurate system for a detailed analysis of the power consumption of a computer at large as well as of any of its components.

Now it’s time to move on to practice. To show the capabilities of the new testing tool and get some practically valuable results I will measure the power consumption of five PC configurations ranging from a cheap digital typewriter to a top-end gaming station.

Office Computer

The first computer is a typical office system. It is an inexpensive computer suitable for office applications:

I installed Microsoft Windows Vista Home Premium SP1 (32-bit) and necessary drivers on the PC.

Let’s start with the very beginning: booting Windows up. The power consumption was being measured from the turning on of the computer to the loading of the Desktop.

As you can see, this configuration has an extremely modest appetite. The current is no higher than 3 amperes on any of the lines. The CPU behaves in an interesting manner: its power consumption is high for the first 20 seconds (the X-axis of the diagram, showing tenths of a second) but then drops suddenly. This marks the moment the ACPI driver is loaded and enables the CPU’s integrated power-saving technologies. After that, the CPU’s power consumption increases above 12-15W only under load.

3DMark06 is obviously limited by the graphics core and cannot load the CPU fully: the latter remains in a reduced-consumption state most of the time. The power consumption grows up a little on the +3.3V line and just a little bit on the +5V line.

The integrated graphics core copes with the heaviest 3D FurMark test easily – in terms of power consumption, that is. Interestingly, the components all show stable consumption although the CPU is not loaded fully: it has a higher consumption at the beginning (the launch of the test) than in the middle of the diagram.

The CPU reaches its peak power consumption – as much as 3 amperes! – under Prime95 (the heaviest test called “In-place large FFTs”). I’m being ironic, of course.

There are no changes when FurMark and Prime95 are running simultaneously. The CPU is loaded fully whereas the integrated graphics core has low power consumption.

So, here are the results.

This computer will obviously do well with any power supply. Even 120W power supplies from mini-ITX cases will ensure a twofold reserve of wattage for such a configuration. The type of load does not affect the power consumption much because the CPU is always the most voracious component. If the 65nm Pentium Dual Core E2220 were replaced with a newer 45nm E5200, the system’s power draw would be a dozen watt lower.

The power consumption in sleep mode (Suspend-to-RAM) is a mere 0.5A (PSUs’ +5Vsb sources can yield 2.5-3 amperes).

Home PC

Next comes an affordable home PC that can be used for casual gaming (the graphics card is not good enough for very heavy 3D games).

I installed Microsoft Windows Vista Home Premium SP1 (32-bit) and necessary drivers on the PC.

Here are power-saving technologies to you: the CPU needs over 50W at the peak but less than 10W at the minimum. The power consumption on the +5V rail also varies greatly – by about 2 amperes.

Note the blue line showing the +12V consumption of the mainboard and the drives: it lowers at the middle of the boot-up process. This is the moment when the graphics card’s power-saving technologies are turned on. In this configuration, the graphics card is powered by the mainboard’s PCI Express slot.

Well, the graphics card and CPU graphs cover the others here. The power consumption of these components is fluctuating constantly because neither is loaded fully (at some moments the graphics card is waiting for a new portion of data from the CPU and at other moments the CPU is waiting for the graphics card to finish the current frame).

By the way, the ordinary “wall outlet” method of measuring the power draw would only show us the average value here, but we can see the full picture.

FurMark loads both the graphics card and CPU uniformly, yet the latter is still not working at its limit, its power draw being but occasionally higher than 3A.

Prime95, on the contrary, puts a heavy load on the CPU but leaves the graphics card without much work. As a result, the CPU is consuming over 60W. The power draw on the +5V rail grows up, too.

The simultaneous run of Prime95 and FurMark produces a uniform load for all the components, the CPU proving to be the most voracious of all.

This voracity is not alarming, though. The whole PC needs only about 137W in the hardest operation mode.

File Server

While it is more or less clear with graphics cards, many users want to know what power supply they need in order to build a RAID array. To answer this question I took the same configuration as in the previous section and added three Western Digital Raptor WD740GD hard disk drives. These are not very new and not very economical of HDDs. They were connected to the chipset’s controller and united into a RAID0.

I installed Microsoft Windows Vista Home Premium SP1 (32-bit) and necessary drivers on the PC.

The disk subsystem was loaded by means of a special program of our own writing. We had written it a few months previously for quite different purposes.

FC-Verify can create and read a specific file set in two independent threads. Thus, at any given moment, there is one read thread and one write thread, which is quite a serious load on the disk subsystem. The files are accessed using Windows API; file caching is disabled; the data block size is 64KB. Besides, the program verifies if the files are read and written correctly, but that’s irrelevant for my purpose. There is a 10-second pause between writing and reading in each thread. The files are deleted after each write-read cycle, and the cycle is repeated from the beginning.

I made the program process a thousand 256KB files in one thread and a hundred 10MB files in the other thread as you can see in the screenshot. The computer’s power consumption was being measured continuously through a few write-read cycles.

But first goes the system boot-up stage with one disk only: the system disk is working while the Raptors are as yet turned off. The diagram does not show anything exceptional besides the longer time it took to enable the CPU’s power-saving technologies. This was because the chipset’s RAID controller took a long thought over the identified system disk and the turned-off array.

This is the same load but the RAID0 array out of three Raptor WD740GD disks is turned on. The most interesting thing is the tall peak at the beginning of the graph that corresponds to the HDDs’ spinning up their platters. The total consumption from the +12V rail (CPU, mainboard and drives) is over 11A then.

It is the +5V rail that is loaded the most here. Clearly, this is due to the HDDs’ electronics as well as to the chipset’s South Bridge the integrated RAID controller resides in.

It is also interesting that the +5V rail is under the biggest load with the RAID array, too. This can be explained, though. A movement of a disk’s head provokes a short spike of current on the +12V rail, but these spikes do not affect the resulting graph much because the array’s three HDDs do not move their heads in sync. Anyway, the diagram shows this in a clearer way.

Somewhat surprisingly, the hardest moment for a file server is when the spindles of all the HDDs are spinning up simultaneously. In the process of normal operation the HDDs’ electronics load the +5V rail considerably while the +12V consumption is rather low.

Thus, a typical 300W power supply is quite enough for powering a 3-disk RAID array with rather voracious HDDs. Such a PSU will easily start the system up and will ensure a threefold reserve of power at work.

I can also note that each fast HDD requires an additional 3.5A on the +12V rail at startup. For large disk arrays based on Raptor-like HDDs it would be good to have a smart RAID controller that can turn HDDs on one by one when the system is started up.

Gaming Computer

The next configuration is a mainstream gaming PC. It allows playing most of today’s games at good settings and has a reasonable price tag:

I installed Microsoft Windows Vista Home Premium SP1 (32-bit) and necessary drivers on the PC.

As usual, we can witness the CPU and graphics card turn on their power-saving technologies – at seconds 5 and 12 of the boot-up process, respectively. The computer is good and boots up fast. Thus, the lack of load does not mean silence and low power consumption: the graphics card and CPU both depend on the driver in this respect.

There is one new graph in the diagram showing the consumption of the graphics card’s additional power connector.

The graphics card’s power draw is changing quickly and greatly. The current on the additional power connector may drop below 4A and then rise up to over 7A. The CPU takes it easy: judging by the power consumption graph, it is idle most of the time.

Interestingly, FurMark provides a very high average load for the graphics card but without such 7A peaks as we have seen under 3DMark. However, the combined +12V consumption is higher under FurMark than under 3DMark because of the higher consumption of the CPU.

The graphics card takes a rest under Prime95. There is a current of only 1A on the additional power connector. The CPU does not consume much, though. Its consumption is no higher than 50A although this number includes the loss on the CPU voltage regulator.

When FurMark and Prime95 are running simultaneously, we have the maximum power consumption of the system. As you can note, the graphics card consumes more than the CPU, especially as a couple of amperes of the blue graph refers to the graphics card, too. I mean the power it receives from the mainboard’s PCI Express slot.

However, the total power draw of the computer is rather modest: 189W. A 300W power supply will ensure a 50% reserve of wattage, and there is absolutely no point in purchasing anything better than a 400W PSU for this configuration.

High-End Gaming PC 1

Next goes a top-end and expensive gaming system based on Intel’s newest processor Core i7.

If you ask at a hardware forum about the power consumption requirements of this configuration, you will most likely be advised to get a 750W power supply at the very least. Will the 500W PSU cope? Let’s see.

I installed Microsoft Windows Vista Home Premium SP1 (32-bit) and necessary drivers on the PC.

We don’t see anything special here. As you could guess, the Core i7 and GeForce GTX 260 have power-saving technologies, too.

Whatever CPU you may get, a fast graphics card will easily beat it in terms of power consumption as we can see here. The power consumption of both the CPU and the graphics card varies wildly under 3DMark06, by a few amperes.

The graphics card’s power consumption under FurMark is changing with a period of 6 or 7 seconds. I cannot explain this effect. It must be this benchmark’s peculiarity. The CPU is under a constant but not heavy load, its power consumption being not higher than 3A (36W) throughout the test.

Prime95 is quite a different story. The graphics card does nothing while the CPU’s power draw jumps from 20W in idle mode to 120W under load! Intel should be given credit for such efficient power management in its CPUs. And I also hope that the upcoming 32nm models will be more energy-efficient under load than today’s 45nm ones.

When Prime95 and FurMark are both launched, the CPU is overloaded (Prime95 is running in as many as 8 threads: four physical CPU cores plus four virtual cores provided by Hyper-Threading technology) and cannot feed data to the graphics card quickly enough. As a result, the graphics card renders one frame and stands idle for a while, its power consumption dropping down.

Here, the “wall outlet” method of power consumption measurements would yield an average much different from the maximum you can see in the diagram above. Of course, the number of Pirme95 threads can be selected in such a way as to ensure optimal work of FurMark and the graphics card, yet it is handier and more reliable to use well-designed measurement tools that yield maximum, minimum and average values together – all in a pretty full-color diagram (you can select the colors of the graphs to your taste!).

The power draw of such a top-performance system is very modest, though. The maximum is only 371W. A 550W power supply will cover its appetite with ease.

As opposed to the previous systems, the consumption of the standby source is nearly zero when the PC is turned on, but it grows up to 0.7A when data is stored in the system memory (in S3 or Suspend-to-RAM mode).

High-End Gaming PC 2

The most serious gaming station is the same as in the previous section but the graphics card is replaced with a dual-chip ASUS ENGTX295 (i.e. GeForce GTX 295). Here is the full configuration:

I installed Microsoft Windows Vista Home Premium SP1 (32-bit) and necessary drivers on the PC.

It is easy to see the moment the ACPI driver is loaded and the CPU’s power-saving technologies are enabled: 15 seconds into the test (the 150 mark of the X-axis). It’s different with the graphics card, though. The consumption on one of its power connectors dropped 30 seconds into the test, but the +3.3V consumption grew up at the same moment. This must be only due to the GTX 295 because the previous system, which only had a different graphics card, did not have such a change in the graph. The power draw on both of the card’s additional power connectors grew up 40 seconds into the test. The power consumption of the mainboard grew up, too, and this addition can only be attributed to the graphics card consuming from the PCI Express slot.

Thus, you should not hope for the GTX 295 to be comparable to single-chip cards in terms of power consumption even when displaying Windows’ Desktop. For more details about that, refer to our graphics card articles.

3DMark06 cannot ensure a constant high load for a modern gaming PC: the power consumption of the graphics card and CPU is varying wildly.

Well, we’ve got FurMark for drawing pretty-looking graphs. Take note of the growth of the power consumption throughout the test: it is due to the CPU getting hotter.

Prime95 makes the CPU consume more than a hundred watts like in the previous configuration. The slanting graph is again explained by the temperature rise: the higher the temperature, the higher the power consumption of an electronic chip.

Take note that the graphics card takes about 3A from the additional power connectors when displaying the Desktop. And the mainboard and drives consume about 5A more from the +12V rail. For comparison, these numbers were 2A and 4A with the previous configuration that had a different graphics card.

When launched simultaneously, FurMark and Prime95 produce a familiar picture: the CPU is overloaded and cannot feed data to the graphics card quickly enough.

To check out how this might affect the “wall outlet” measurement, I took the PM-300 wattmeter mentioned at the beginning of the review. It reported a maximum of 490W. Considering the 90% efficiency of the PSU, it means that the computer consumed 441W from the PSU. But my testing tool reports a maximum consumption of somewhat higher than 500W. This big difference is due to the wattmeter’s reporting an average rather than maximum value when the power consumption is so fluctuating.

Of course, my testing tool can also calculate the average that is indicative of the system’s heat dissipation and of your electricity bill. But you want to know the maximum power draw in order to choose an appropriate power supply.

It is still unclear who needs those 1000W power supplies because a 750W unit is quite enough even for this very top-end configuration. A 1000W PSU is two times the required wattage, which is obviously redundant.


To sum everything up, I will show you a summary diagram with two values for each PC configuration: maximum load (FurMark + Prime95) and typical load (3DMark06).

So, there is nothing frightening about the numbers. Of course, 500 watts is quite a lot. It is about one quarter of an electric iron, but PSUs that can deliver it are widely available for reasonable money, especially if you compare it with the cost of the other components of such a power-hungry configuration. If you want to have a 50% reserve of wattage, a 750W power supply will be sufficient for a system with a Core i7-920 and a GeForce GTX 295.

The other configurations are much more economical. If the graphics card is replaced with a single-chip one, a 500-550W power supply can be used (and it will have a reserve of wattage, too). And an inexpensive 400W PSU will do for midrange gaming PCs.

Note also that this is the power consumption under very heavy tests. No real game can load the computer as heavily as FurMark. It means that a 750W PSU will offer an even larger reserve of power for the most advanced of the tested configurations.

Talking about the new measurement tool, it covers all of our test lab’s purposes and allows to measure the overall consumption of a computer as well as that of any of its components at any moment, starting from your pressing the Power button or even before that. It can automatically measure the minimum and maximum of currents, find an average power consumption, calculate maximums of power consumption (considering that you can’t just sum up the maximums on the different PSU rails because these maximums may have occurred at different moments), show the distribution of load among the different PSU rails and draw graphs that show how load changes over time.

Such tools will soon be used in most of our reviews, configured specifically for the particular purposes. For example, in this review I totaled the consumption of the mainboard and drives, but in graphics card reviews the current the card consumes from the mainboard’s PCI Express slot can be measured separately.

And to make our PSU tests more illustrative, we will show the power consumption levels of different PC configurations in cross-load diagrams. We once did so, but were limited then by the lack of a handy measurement tool.