Recently, there has been a surge of topics on this forum that inquiries, “Is my ___ PSU capable of powering ___ number of ___ GPU with ___ overclocks on my ___ CPU?” The truth is, it really is not that complicated at all to figure out the power requirement of a computer as long as you know what is in it. This post will hopefully help you learn how to calculate this by yourself.
Always calculate the maximum
amount of power you need and buy a PSU that can handle the maximum
power you need. A larger PSU will only supply as much power as your system need – not forcing its maximum capability upon the system. On the contrary, insufficient power will cause instability. In the worst case scenario, you will have hardware failure due to overheating as your system forces the overwhelmed PSU to pull out more power than the PSU is designed to.
Since how much power a processor uses depends on mostly the frequency and voltage, the processor is what throws people off the most since overclocking changes both of that. To top it all off, every chip is different, and there is no definite setting for the voltage between all the chips.
Though, there is a set variable in the process of determining processor power usage, and that is the thermal design power, or TDP. As a very general rule, the TDP of a processor on the specification sheet is the estimate of the maximum power the CPU uses at stock settings.
However, do note that the TDP does not define
the power usage, for it actually defines the maximum thermal dissipation in heat. In fact, the TDP is a thermal reference - not an electrical one.
There is a formula that estimates power usage by the CPU:
OC wattage = TDP * (OC frequency / stock frequency) * (OC core voltage / stock core voltage)2
Unfortunately, electronics cannot be 100% efficient in real life - so you have to also factor in efficiency. Netburst (e.g. Prescott, Cedar Mill) is about 8% efficient, Core 2 is about 10% efficient, and Nehalem is about 12%. What happens to the rest (92% in Netburst, 90% in Core 2, and 88% in Nehalem)? Let's just say that is why the processors get hot.
To implement this, you can use:
Final OC wattage = OC wattage / (1-efficiency)
Let us work on an example: given an i7-930 overclocked to 4GHz (4004MHz), what would be the maximum power usage?
As stated before, every chip is different – both the stock core voltage and overclock core voltage will have variations between chips. Therefore, 1.215V will be used as the stock core voltage, and 1.35V for the overclock core voltage. Obviously, the given stock frequency of 2800MHz, OC frequency of 4000MHz, and TDP of 130W will be used.
OC wattage = 130W * ( 4004MHz / 2800MHz) * ( 1.35V / 1.23V)2
= 130W * 1.430 * 1.205
= 130W * 1.723
Therefore, the maximum power this particular i7-930 will use is about 224W in the perfect world. However, this world is not, so we must factor in efficiency.
Final OC wattage = 224 / (1 -. 12)
= 224 / .88
In real life, with efficiency factored in, the i7-930 in this particular situation will consume about 255W.
I cannot stress enough that the above example is a rough approximation given for the given situation only
! Even in this situation, the estimated power usage is actually higher than it is. There are three main reasons for the aforementioned phenomenon.
1. The stock core voltage and overclock core voltage may be very different for another case.
2. The stock frequency and overclock frequency may be slightly different for another case.
3. The TDP on the specification sheet is not exact; it is actually higher than what it is. Processors are given TDP by ranks, such as 65W, 92W, 95W, 105W, and 130W. The real TDP of the chips fall between those categories, and the TDP given on the specification sheet is the closest TDP category above the real TDP.
Here is a list of Intel desktop chips and their TDP categorization for easy reference:
II. Graphics Card
If you do not overclock your graphics card, all you need to do is find the maximum power usage on the website of the chip-maker (e.g. NVIDIA, AMD, Matrox) or vendor (e.g. EVGA, Sapphire, etc.) However, do be careful to do research on the actual
power consumption of these cards, which may be lower or higher than the published numbers. The GTX 480, for example, have been seen to suck up as much as 300W at testing, as opposed to the 250W of “maximum board power” given by NVIDIA.
If you do overclock your graphics card, do leave about 20W to 60W more per card, depending on how much overclocking was done. For example, the EVGA GTX 480 HC FTW Edition uses about 50W more than the reference card.
III. Almost Everything Else
This is the easiest part of the whole step because most of these approximations are set in stone.
These are the general guidelines at maximum load:
The motherboard is a rough estimate. The data given was the data from the most energy-hungry of the bunch – 780i and X38. Likewise, everything else is taken from the worst of the worst. It was only done because headroom is greatly under-appreciated. Most other peripherals (i.e. sound card) will use so little power, that the (over)estimation done so far probably covered it up already.
In the end, it is equally important for you to pick out a quality power supply unit – that means you have to look beyond the paper label! Check around for reviews, especially on JonnyGURU. A PSU Guide
by boredgunner and another one
by myself have recommendations of good power supply units.
Hopefully, you now know how to calculate your system power usage. Do not underestimate the power needs of your system!
EDIT (September 1, 2010): Fixed messed up table.
EDIT (September 8, 2010): Added efficiency and emphasized on TDP's thermal implications. Thanks lehpron.
<message edited by cai_shawn on Tuesday, July 31, 2012 3:04 PM>