If you frame a desktop system as a body, the computer power supply would be the heart. But instead of pumping plasma, it takes power from the wall and converts it to be used by the hardware components. Whether it is for a new system build or replacing a broken unit, getting the right power supply is important. But finding the right one can be a challenge, as looking at the spec sheet for a PSU can be like staring at a bowl of alphabet—or alphanumerical—soup.
TDP? Watt is that? Thermal Design Power
Wattage lies at the heart of the matter. Every component in the computer has its own wattage requirement and if your power supply doesn’t supply enough of it, the computer might not function properly or turn on at all.
All major components such as the CPU, video card, and motherboard have ratings for the maximum amount of power they draw, referred to as thermal design power or TDP. Manufacturers usually–but not always–provide TDP numbers for their hardware. The first step in buying a power supply is to collect all the wattage requirements you can for the hardware.
Once you’ve found your components’ TDP ratings, add them up to get your system’s total power requirement. Your power supply should at a minimum meet that number or preferably exceed it by 20%. For example, if your system requires a total of 500 watts, get a power supply that can provide 600 watts. Keep in mind that price scales with wattage, so the more wattage you need the more you should expect to pay.
Note the Continuous Power Rating, Not the Peak Power Rating
When looking at a power supply’s wattage, determine whether it is the peak rating or continuous rating. Peak tells you how much power the PSU can provide, regardless of sustainability–it could be one minute or one hour. Continuous tells you much power it can supply consistently. Be wary if a power supply only has a peak power output rating and none for consistent output.
Making a Connection: 20- and 24-pin Motherboards
Once you have determined how much power your computer requires, find out what type of internal power connectors your computer’s hardware uses. Motherboards can have either 20-pin or 24-pin power supply connections, though newer models typically use a 24-pin connector. Make sure the motherboard and power supply are on the same page when it comes to 20- vs 24-pin.
Apart from the motherboard, you also have video cards and other PCI Express (PCI-E) devices that may require specific connectors because many of them draw power separately from their motherboard slot. It gets tricky because PCI-E devices can require either 6-pin or 8-pin cables. To get around that, higher-end PSUs often have 6+2 pin cables that fit both 6- and 8-pin devices, such as the one pictured below.
Tidy that Mess with Modular Cabling
If you’ve ever compared the inside of a computer with modular power supply cabling to one without, you’ll almost immediately notice the difference. Modular power supply cabling allows you to disconnect unused cables to reduce cable clutter. Without it, the inside of your computer can look like a mess of wires at best and have terrible airflow at worst. However, modular power supplies do tend to cost more on average however.
Will it fit? ATX, Micro ATX, and Mini ITX form factors
The vast majority of computer cases and power supplies support the standard ATX form factor. But certain application-specific and mini-computers require non-ATX components. For instance, smaller systems may require a Micro ATX or Mini ITX power supply. Be sure the power supply you purchase physically fits the case you want to install it in.
80 PLUS Certification
Those Bronze, Silver, and Gold 80 PLUS certification ratings serve as indicators of overall power supply efficiency. The higher the rating, the more efficient the power supply operates. For an in-depth explanation of the 80 PLUS system, see our power supply certification guide.
Power Supply Life Expectancy
The standard metric used to measure power supply life expectancy is the mean time between failure (MTBF) rating. Unfortunately, it there is no standardized testing methodology and it is only an approximation. For instance, power supplies are susceptible to wear due to heat. Now did a manufacturer get a MTBF number through testing the unit at a normal operating temperature or an abnormally cool one? Some question the reliability of MTBF numbers in general.
The best way strategy is to choose a power supply with a MTBF rating that explicitly states what temperature it was achieved at. For example, this Rosewill 750 watt (continuous) power supply achieved a 100,000 hour MTBF at 25 degrees Celsius. Shop wisely and look for a power supply that can achieve a 100,000 hour or greater MTBF at operating temperatures you expect it to see.
Overvoltage/Overcurrent Protection: What it Means
If the output voltage exceeds a specified limit, the power supply shuts down. This protects the computer hardware from damage. Most power supplies have this, but some budget-minded offerings may lack this feature.
How much should I pay for a power supply?
As much as you need for the feature set you want. If you want to minimize cost, look for a power supply that meets your wattage requirements and matches your device connection needs and that’s it. Forget about modular cabling, overvoltage protection, 80 PLUS certification, and a detailed MTBF rating.
Take an inventory of your power supply needs and get the features you must have. Know that higher wattages and advanced features like modular cabling will drive up costs, so shop accordingly. If you only need 500 watts, don’t buy a top-of-the-line 1200 watt unit with modular cabling, over voltage protection, and 80 PLUS GOLD certification.
What else makes a good power supply?
Another good metric is the percentage a power supply deviates from its rated voltages. A good power supply should stay within 3% of its rated voltages at all times, with some top end models deviating only 1.5%. Look for this rating under voltage or load regulation, assuming it is provided. Not all manufacturers provide voltage regulation numbers.
Summing it all up
Here are the things you want to pay attention to when selecting a computer power supply: wattage, internal connectors, form factor, modular cabling (yes/no), efficiency ratings, MTBF, voltage regulation, and overvoltage protection. You may find Cooler Master’s power supply calculator useful for estimating your system’s total power requirement. Searching for a computer power supply need not be a confusing experience, just be sure to pay for the features you need and not the ones you don’t.