Everything you need to know about power supplies

Efficiency is the second most important feature to look for after output power. A PSU’s efficiency rating is effectively a measure of how well it performs. In other words, how much energy is wasted in the conversion process. A PSU with 80 per cent efficiency will pull 500W from the wall to deliver 400W for your system, while a PSU with 70 per cent efficiency will need to pull 570W from the wall to do the same. On the surface this means a more efficienct PSU is slightly cheaper to run, but there’s a more important benefit: wasted energy is lost as heat, and heat is the number one enemy of a PSU.

As a PSU gets hotter it becomes less efficient and its maximum output power drops. In turn, its efficiency drops and the process spirals. Additionally, if it gets too hot, it becomes harder for the PSU to maintain the rails within their regulation (usually +/- five per cent) and since your gear is developed with these specs in mind, the result can be instability or crashes.

This loss in performance as a PSU heats up is known as the de-rating curve, and is the reason a PSU is rated for operation at up to a certain termperature. A 500W PSU rated for 50 degrees Celsius is rated to provide 500W at up to 50 degrees, past this its maximum output can drop. Considering the insides of most cases can usually get rather warm, there’s a good reason PC PSUs are held against the 50 degrees threshold.

Naturally as it gets hotter the PSU’s fan will spin faster in an effort to expel the heat, and it’s not a lost irony that if it’s getting hot due to a high load then it’s usually pulling in hot air from inside the case as well.

Today any good PSU will be branded with the ‘80 plus’ sticker that shows it can maintain 80 per cent efficiency for its typical load (usually 50-75 per cent of its rated power). It’s true that some PSUs lose efficiency at low power loads (say, 20 per cent of power) as well as at very high loads (especially when reaching 100 per cent or more). Naturally, if efficiency drops, more heat is generated, compounding any heat issues. These days, however, more and more PSUs are increasingly able to deliver 80 per cent efficiency across all loads – but check when you buy to see if this is the case.

It’s for this reason that it’s recommended to get a PSU with at least 30-40 per cent more power than you think you’ll need under load. This isn’t about headroom for future growth (you should’ve taken that into account already), it’s about ensuring you don’t approach the PSUs maximum output power to ensure maximum efficiency and minimal heat. The secret to a stable and quiet PSU is a higher output power and a high efficiency.

But even though 80 per cent efficiency is rather good (PC PSUs used to be 70 per cent and less only a few years ago) wouldn’t it be better if it could be 90 per cent? Indeed, a push is already under way in the industry to see a new 90 per cent standard introduced, and we may well see ‘90 plus’ PSUs in the next few years.