Table of Contents
Power Supply Wattage
Different components inside a computer run off a variety of voltages: most commonly 12V, 5V, and 3.3V. A power supply has individual ratings for each of those levels – as seen in the image to the left. Because of that, the total wattage a power supply can output is limited in part by those separate maximums. They can also be an issue on their own, in rare situations, if a system has an abundance of parts which all need a single voltage.
This leads to the first recommendation for picking a power supply: you want one which is capable of supplying more power than your internal components will need to draw; rather obvious, it is true, but worth stating for the record. Depending on the exact hardware being used you may also need to make sure that none of the individual voltage rails will be overwhelmed, but that is usually only a concern on very high-end systems.
Power Supply Efficiency
To encourage use of high efficiency power supplies, and help ensure fair comparisons between models with different efficiency, the 80 PLUS initiative was started. It offers the option for manufacturers to have their power supplies certified by a third party, and rated on a scale based on efficiency; power supplies that have been through this process are listed on their website. The ratings for desktop computer power supplies are as follows:
|Certification \ Fraction of Rated Load||20%||50%||100%|
|80 PLUS Bronze||82%||85%||82%|
|80 PLUS Silver||85%||88%||85%|
|80 PLUS Gold||87%||90%||87%|
|80 PLUS Platinum||90%||92%||89%|
As can been seen in that chart, most power supplies offer their highest efficiency when providing an amount of power in the middle of what they are able to handle. As you go to the farthest ends of a given power supply’s wattage range efficiency drops off, sometimes noticeably. With that in mind, it is best to aim for a power supply which is not only capable of handling the maximum draw of the combined hardware in your computer, but one which exceeds that amount enough that to help maintain near-maximum efficiency even under heavy workloads. The inverse is also true: you don’t want a power supply which will drop too far below its maximum efficiency when the system is idling… though that is less important than higher load levels, because efficiency matters less the lower the power draw is.
The Impact of Efficiency
According to my last bill, a KWH where I live costs me 10.4 cents, so by having the more efficient of those units I’d be saving about a $1.25 per month. That may not sound like much, but that is $15 a year – and $60 if I used the system in that fashion for a period of four years (a reasonable life for a hard-core gaming system). That is also a relatively conservative estimate in that I’m sure some parts of the U.S. have higher energy costs than here in the Pacific Northwest, where hydro-electric dams are plentiful.
As you can see, efficiency has the potential to save a lot of money in the right circumstances. Systems that are going to be running 24/7, like servers and home theater systems, can show even greater returns on investment. On the flip side, though, high efficiency models tend to also cost more – so, like most decisions on computer hardware, it is a balancing act.
Other Considerations when Choosing a Power Supply
Reliability / Quality – The power supply is one of the few items in a computer that has the potential to damage other components if it fails. It is also one of the least glamorous, as it doesn’t impact performance – so long as it is working – so some folks don’t pay attention to the quality of the model they get in a computer. That can be a fatal flaw, though: if the power supply fails then the system won’t work until it is replaced, and as alluded to above they can sometimes die in spectacular ways, taking other parts of the computer with them. Looking at brand names and reading reviews can certainly help make a good decision, and here at Puget Systems we keep records of all part failures we encounter or customers report to us so that we can make sure we only sell reliable parts.
Cooling / Noise Level – Almost all power supplies are actively cooled, meaning they have a fan (or sometimes two). The size and type of fan used can affect how much noise the power supply makes, and the way the power supply is laid out and where the fan is located determine how well the unit is cooled. A poorly-placed fan can mean less airflow to critical parts inside the power supply, meaning the fan has to spin even faster – and louder – to cool things effectively. Efficiency also comes into play here: when electricity from the wall isn’t turned into usable power in the computer it is lost as heat, so less efficient power supplies will run hotter and need more cooling.
Cabling / Power Connectors – Various parts inside a computer require different types of power connections, so the power supply for a given computer needs to provide enough of each type for that specific system’s hardware. Thankfully this is rarely a problem with modern power supplies, but there are a few things to watch for: making sure you have enough PCI-Express power leads for the video cards in your system (only an issue for systems with high-end or multiple video cards) and ensuring you get a power supply with EPS power connectors if using a dual CPU motherboard (some servers and workstations) are the two most common things I’ve run into. In addition to the connectors themselves, many power supplies these days offer modular cabling – allowing installation of only the necessary power leads in a given computer, in turn cutting down on clutter inside the system.