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Power Supply Buying Guide
Table of contents
The power supply (a.k.a. power supply unit or PSU) is essentially the heart of a computer - it provides the driving (electric) power for all essential computer components, such as the processor, motherboard, memory modules, video card and add-in cards, as well as hard drives and optical drives. The computer power supply converts an alternative current (AC, which is obtained from the wall socket) into a specified voltage direct current (DC, which can be used by computer components).
Many users do not treat the computer power supply very seriously as it usually does not provide exciting new features or direct performance improvements to a computer system. The truth is that the power supply is a vitally important part of a computer system as every component inside the computer case is fed by the PSU. A high quality power supply with enough power capacity ensures the functionality of your equipments as well as their reliability and durability. On the other hand, many problems and malfunctions are caused by low quality or under-powered PSUs. For instance, a defective power supply can result in random system resets/freezes.
Knowing the importance of the power supply is the first step in the quest to find the right PSU for your computer system (this applies to desktop computers throughout this article). This means much more than 'the more watts the better', as we will discover in this buying guide.
Like motherboards and computer cases, there are several different power supply form factors, sizes, connector types, output specifications and other important specs. The detailed differences between these power supplies are extremely important and can make the difference between the ability to run a computer at its full potential and having a potentially unstable computer.
Although there are still AT form factor power supplies available for purchase, AT form factor power supplies are undoubtedly phased out products. Even the later ATX form factor power supply (ATX 2.03 and earlier versions) are falling out of favor. The major differences between the ATX and AT power supply form factors are:
1. ATX power supplies provide an extra +3.3V voltage rail.
2. ATX power supplies use a single 20-pin connector as the main power connector.
3. ATX power supplies support the soft-off feature, allowing software to turn off the power supply.
The ATX12V form factor is the mainstream choice now. There are several different versions of the ATX12V form factor, and they can be very different from one another. The ATX12V v1.0 specification added over the original ATX form factor a 4-pin +12V connector to deliver power exclusively to the processor; and a 6-pin auxiliary power connector providing the +3.3V and +5V voltages. The ensuing ATX12V v1.3 specification added on top of that the 15-pin SATA power connector.
EPS12V, SFX12V and Others
The EPS12V power supply form factor utilizes an 8-pin processor power connector in addition to the 4-pin connector of the ATX12V form factor. (Note: this isn't the only difference between these two form factors, but for most desktop computer users, knowing this should be sufficient). The EPS12V form factor was originally designed for entry-level servers, but more and more high-end desktop motherboards are featuring the 8-pin EPS12V processor power connector now, which enables users to opt for an EPS12V power supply.
Many of us typically begin and end our search by looking at one index of a power supply's performance: its maximum output. We then judge different power supplies using only this figure. This is definitely not the recommended approach as choosing a power supply that suits your system's power requirements is more complex than just comparing output figures. In this section we'll take a look and gain a basic understanding of the most important power supply specs/features and what they do.
Power supplies are basically differentiated by output wattage, which is quoted according to the "Maximum Power" output. Maximum power is not just a number, it also gives a basic idea of how many devices (and what type of devices, in terms of power consumption) the power supply will be able to feed.
Please note that continuous power and peak power are different. Generally, the "Maximum Power" figure of a power supply refers to the continuous (stable) power the PSU will deliver consistently, while the peak power refers to the elevated maximum (surge) power the PSU can deliver, albeit for very short amount of time (e.g. 15 seconds).
The connectors provided by your power supply determine the types of devices that can be connected to it. Therefore it is extremely important that you choose a PSU with all the connectors (type and number) your components require.
There are other power connectors provided by the PSU, including floppy drive connectors (a.k.a. mini connector), PCIe connectors (for high-end video cards, 6-pin) and the two types shown below.
PFC (Power Factor Correction) is a technique that counteracts the unwanted effects of electric loads (reactive power) that make the power factor less than 1. The Power factor (PF) refers to the ratio of active power (measured in watts) to the apparent power (voltage multiplies current - volts x amps or VA), which includes both active and reactive power, and only active power is capable of doing work. For a power supply unit, the higher the PF value, the better it is able to convert current into useful power.
The efficiency of a power supply unit is the percentage of total output DC power in relation to total input AC power. The portion lost during conversion is mostly in the form of heat.
No power supply is currently able to reach 100% efficiency, which means there will always be heat produced during operation. Generally, a higher output PSU will result in more heat being produced. Most power supplies use one or more fans for cooling.
Modular cabling allows a user to choose the type and number of power connectors (mainly peripheral and SATA power connectors, and PCIe) to be used. The picture below shows a power supply with modular cabling features - it provide a main power and processor power connector by default as almost every system will require these two connections. The other power cables (usually provided with the PSU) need to be hooked up to the available power sockets as needed.
Overvoltage protection refers to a circuit or mechanism that shuts down the power supply unit if the output voltage exceeds the specified voltage limit, which is often higher than rated output voltage. This protection is important since high output voltages may cause damage to computer components that are connected to the power supply.
Similar to overvoltage protection, overcurrent and overload protection are circuits that protect the power supply unit and the computer by shutting down the power supply unit when there is excessive current or power load detected, including short circuit currents.
Important Note: Please begin by selecting a PSU that supports your motherboard connector type (20 or 24-pin) and provides enough power connectors to connect all your devices.
Component Typical Power Requirements
For most home and office users, a 400W power should be more than enough - but that’s not always true if you have power hungry processors/video cards or many connected devices. If this is the case, please be sure to look at the above table and calculate your rough power requirements.
Multimedia/Large Storage User
If you have lots of storage devices, such as hard drives, optical drives or other devices, a power supply with at least 450W is a necessity.
Note: Please refer to the "Component Typical Power Requirements" table and calculate your specific maximum power requirements.
For gamers with SLI or CrossFire configurations, a power supply rated more than 500W is a minimum requirement, with 550W ~ 600W recommended. Please make sure the +12V rail(s) provide no less than 34A (combined) when you are going to run a high-end SLI/CrossFire system.