Vito Cassisi – Tech Blog Delving into technology

Efficiency Deficiency

Wattage, price and efficiency - why low quality PSUs are not going to cut it.

When designing a PC, components which affect performance are typically assigned higher priority. For instance, the CPU, GPU and RAM. Enthusiasts will also ensure that the motherboard is of high quality, has an array of expansion capabilities, good overclockability, and will perhaps spend extra on a decent CPU cooler. Next up are the HDDs and possibly a pretty case. What often gets left to the bottom of the priority list are optical drives and the humble power supply unit (PSU). Professional PC designers, however, will place the PSU amongst the high priority components. The PSU is the difference between a functional PC, a PC that constantly crashes, and a PC that turns to a crisp during a power surge. In fact, it's one of the most important choices during the design stage.

I think it's safe to say that many people have had a PSU die on them before, or know of someone that has had it happen. Sometimes the fried component will only be the PSU. Other times the PSU fails to protect the components it is connected to, and causes them to fry like a potato chip in hot oil. The difference between the two lies with the PSU itself. A quality PSU can be the difference between replacing $150 worth of gear, and $1000+ worth.

Of course, PSU failure isn't the only situation of which a quality unit is advantageous. A quality power supply unit will be able to supply power under differing loads, whilst maintaining a high efficiency. Computers with components that require more power need a larger wattage PSU to supply it. Cheaper, lesser quality PSUs often claim similar wattage's to units which are of a significantly higher price. Most consumers don't consider the reason why this may be, after all, a larger wattage must be better. However, this isn't always the case. We must consider the efficiency of the PSU. To put it simply, efficiency describes how effectively a transformer (i.e. the PSU) can transform AC power (from a wall socket) to the DC power which is required by the computer components. PC PSUs also step down the mains voltage from 240V to 12v, 5v and 3.3v.

PSUs are not 100% efficient - power is lost via heat during the transformation phase. To calculate the amount of power a system will lose to heat, we must know the efficiency percentage. Let's assume that the aforementioned PSU is 80% efficient:

input x .80 = 500W
input = 500/.80
input = 625W

As you can see, a PSU of this calibre will lose 125W of power to heat. The PSU will draw more power from the mains to compensate. In this case, 625W.

Why is this a bad thing? Heat causes components within the PSU to reduce in lifespan. In addition to this, PSUs can only perform optimally under particular temperature conditions. The hotter a PSU gets, the less power it can supply to the PC. This is called the PSUs 'Derating Curve'. Basically, for every degree the PSU raises in temperature, a particular amount of output wattage is lost. As the PSU gets hotter, the rate at which power is lost becomes exponential (which is a very bad thing). More information on derating curves can be found here.

So what efficiency should you be looking for in a PSU? The general rule of thumb is to get an 80 PLUS certified unit. These PSUs are guaranteed to be at least 80% efficient under load. 80+ certification is split into four categories, standard, bronze, silver and gold. A table which explains the difference between these categories can be found here. The 80 PLUS website displays a list of PSUs which meet the these standards.

An added advantage to high efficiency power supplies is the reduced heat as a result from the transformation process. Less heat means a quieter fan, or in some cases, no need for a fan at all. If the PSU is a cheap one, which is probably the case if it's not very efficient, it'll run very loud in an attempt to cool itself.

Efficiency isn't the only aspect that makes a good PSU. Line noise is another issue which must be considered. A PSU with large amounts of noise may cause system instability whilst under load. This is particularly critical when overclocking a machine. Frustration often occurs when hardware errors are constantly thrown by the computer, and the source of said issues are hard to diagnose. Such errors may well be a dodgy PSU. This is why its preferred to buy a quality unit from the very beginning.

All in all, it's best to spend extra on a quality power supply. Checking out review sites which use proper PSU testing equipment is the best way to check the suitability of a unit. These sites include SilentPCReview and Hardware Secrets. You'll get the advantage of a quieter and cooler unit with higher output, less line noise, and significantly better longevity. :)

Edit: Re-wrote explanation of 'efficiency' due to error. 3/07/09