With a top of the range power supply unit (PSU) rated up to 1200W a computer builder could be forgiven for thinking that a chunky PSU was needed to power their system. After all we all like to think we have the best and most powerful system around. The truth is that even very power hungry systems consume far less than this. So how do we calculate how much power we really need? There are two options. We can head over to an on-line calculation tool such as Extreme Outervision or we can do the job ourselves. Both of these approaches benefit from a basic understanding of how a system uses the output voltages of the PSU. Modern systems conform to the ATX12V standard. An ATX12V PSU provides the following voltages; +3.3V, +5V, -12V and +12V. From the table above it is clear that 12V is the most widely utilised of the voltages available from the PSU. It is also the voltage that is used by the CPU & GPU which between them consume a majority of the power supplied to the system. It is therefore reasonable to use the 12V requirements to determine the capacity of power supply required. When estimating the capacity required in a PSU it is important to realise that it is only an estimate, not a precise calculation and that the power consumption of a system varies considerably depending on the components that make up the system and how the system is used. Even if you are building a top end gaming rig there are times that it will be used for tasks such as web browsing which does not make high demands on it and therefore the load on the PSU will consequently be very low. The approach I prefer to use to estimate the load capacity required from the PSU is to estimate the peak load required on the 12V rail(s) and match that with 80% of the continuous rated output of the 12V rail(s) of a PSU. Why 80%? Firstly the efficiency of a PSU is still reasonably high at 80% load, particularly if it is 80plus certified. It provides a margin between the maximum required load and the maximum the PSU can supply. This means the PSU is not working as hard and is more likely to last longer than one run at full capacity, particularly as the performance of the PSU deteriorates with age. Computer systems do not constantly draw peak load, even when running applications such as 3D games. Therefore by matching the peak load of the system to 80% rated capacity of the PSU it is probable that the average system load will be well matched to the maximum efficiency of the PSU which is typically closer to 50% of rated load. It reduces the likelihood of the efficiency of the PSU dropping away too much when the system is idling. Higher rated PSUs cost more therefore by better matching the rating of the PSU to the system requirements will save money. The main exception to this approach would be for systems that require near silent operation. A PSU generates noise primarily from the operation of the cooling fan therefore by reducing the need for the cooling fan to spin up to high revs will minimise the noise. This can be achieved by using a PSU that has a significantly higher output power rating than is required. Calculating System Load The system load can be calculated by listing the components that make up the system, assigning a 12V load value to each component then adding them up. I have listed some typical 12V loads below: HDD assume 15W Fan assume 5W PCI assume < 25W PCI-E (except when used for graphics cards) assume < 10W Chipset assume 10W Memory assume 2W per stick CPU In the absence of any better information I have used the TDP to represent the power consumption of CPUs. Unless otherwise specified the source I have used is Wikipedia “List of CPU Power Dissapation” Phenom II X4 965 – 140W (prior to C3 stepping) or 125W Phenom II X4 955 – 125W Phenom II X3 720 – 95W Phenom II X2 555 – 80W Athlon II X4 630 – 95W Athon II X3 435 – 95W Athlon II X2 250 – 65W Core i7 920 – 130W Core i7 860 – 95W Core i5 750 – 95W Core i3 530 – 73W Core i5-2300 (excluding graphics core) - 51W Core i5-2400 (excluding graphics core) - 55W Core i5-2500 (excluding graphics core) - 62W When the CPU is being overclocked I allow an additional 100W to 150W. It is a bit of a guess really but if you look at reviews from a site that overclocks as part of the review (such as Bit-Tech) you can get an idea bu looking at the difference between the power consumption at stock values and overclocked values. GPU Xbit Labs measure the power used by a graphics card when conducting a review and for that reason I tend to use their power consumption figures where possible. Radeon HD5970 – 203W (stress test), 191W (3d gaming) Radeon HD5870 – 161W (stress test), 107W (3d gaming) Radeon HD 5770 – 81W (stress test), 61W (3d gaming) Radeon HD5750 – 61W (stress test), 44W (3d gaming) Geforce GTX275 – 140W (3d gaming) 55nm Geforce GTX260 216 – 105W (3d gaming) Geforce GTS250 – 81W (3d gaming) Worked Example For the purpose of a worked example I will use the following hypothetical system: Intel Core i5 750 P55 chipset AMD Radeon HD5870 2 x Sata HDD 2 x 2GB DDR3 Asus Xonar D2X soundcard 2 x System fans CPU fan To calculate the system 12V load is simply a matter allocating the estimated power consumption of each component and adding them together to get a system power consumption. 325W/0.8 = 406W We are looking for a PSU with 406W available on the 12V rail(s). So we are looking at a 450W to 500W PSU. A quick check on the specs of a couple of popular PSUs shows that: A Corsair CX500 V2 has 408W on the 12V rail – close enough A 500W Enermax LibertyEco II 500W has 456W on the 12V rails – another option If the Core i5 750 was being overclocked. 325W+150W = 475W 475W/0.8 = 593W Again a check on the specs of a couple of popular PSUs shows that: A Enermax Modu 82+ II 625W has 600W on the 12V rails – certainly an option. A Corsair TX650M has 648W on the 12V rail – another good option. The approach I use is based on using a good quality PSU that can continuously deliver the stated power output. If you are looking for guidance in this area can I suggest taking a look at "Recommeded PSU List"