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Power supply basics



One of the most often overlooked parts in any PC is the power supply. A lot of people used to consider the PSU one of the least important parts of a PC that they preferred to spend as little money as possible on. Others think that when overclocking and installing dual graphics cards they are in need of a huge power supply and race towards the 1000W models.

Our demographic - the enthusiasts - have known for a while that the power supply is actually one of the more important parts in a PC. The parameters by which we choose a PSU however aren't always the correct ones. Time to clear up some of the questions on the subject of power supplies.

To kick off this article I'll start with a list of commonly asked questions. With each answer we'll also dive into related stuff as it comes up.

Does a 1000W PSU use more electricity than a 500W PSU?

No. How much a PSU uses depends on how much the components it supplies power to are demanding. In other words, as long as the power demand stays under 500W both power supplies will use roughly the same amount of power.

However: the two power supplies may have a different efficiency. How efficient a power supply is also varies as the load on the power supply varies so the real answer is not as easy. Suffice it to say though that the size of your electrical bill depends mainly on the components inside the computer and hardly on the Wattage of the PSU.

The reason for not wanting a 1000W power supply when only using 500W is however one of efficiency. The lower the load on a power supply the worse the efficiency generally gets so it is still important for your electrical bill not to use a PSU with a huge overcapacity.

Efficiency, Supply and Demand explained

Until the arrival of the 80 PLUS bronze, silver and gold power supplies most PSUs had an efficiency of around 70%. What this means is that from the power going in to the power supply from your wall socket about 30% is converted in to heat (or: 30% of that energy is lost unless you consider warming up your room to be a goal as well). This leaves 70% of that power available to actually power the components in your computer. One frequently made mistake by lots and lots (and lots!) of reviewers and hardware enthusiasts is that they measure the power consumption of - for example - a videocard by measuring the amount of power their test system draws without the videocard, then go on to measure the power draw with the videocard installed, substract the 2 and give you the difference as being the power draw of that particular videocard. Because they forget to take the efficiency of the PSU into account this number is off between 10% and 30%! Considering the fact that most PSUs these days are about 80% efficient, most of the time this number will be off by around 20%. This means that if their calculation says that a particular videocard is using 300W, in reality it is probably only using 300 x 80% = 240W. For many people this can make the difference of thinking they need a new PSU or not if they upgrade to a new videocard!

A 500W power supply can SUPPLY 500W of power to your components. If this power supply is 70% efficient this means that it would draw about (500 / 70) x 100 = 714W from your wall socket if it's under full load. Even if you are not particularly environmentally conscious it is still a good idea to buy an 80 PLUS power supply because at these power loads your wallet will thank you.

A word of caution on Power Supply Ratings

There is no official rating for power supplies, therefore the Wattage of a PSU is rated by the manufacturer. This is one of several reasons you see a lot of people with severe overcapacity on their power supplies. Some power supplies labeled 500W can deliver a peak Wattage of 500W while other power supplies labeled 500W can deliver 500W continuously. Some will deliver their rated wattage only below certain temperatures.
The higher tier brands tend to specify the continuous power a PSU can supply while some of the lower end power supplies are labeled with their peak Wattage. In addition, some brands have their PSUs made by various OEM factories while other brands manufacture the power supplies themselves. This is the reason that with some brands only certain models are good power supplies while other models have a relatively high failure rate. Oftentimes some models will be made by a different OEM manufacturer than other models. Sometimes this is even the case with models in the same lineup of a particular brand. Consequently, if a 500W series X power supply aces professional tests, this is unfortunately no guarantee that the 600W series X power supply is equally great.

Non OEM brands

Two top brands that are also their own manufacturers are SeaSonic and Enermax. These power supplies are made using high-grade components and usually carry their continuous rated Wattage on the label. Due to the high-grade components and usage of the continuous Wattage these power supplies may appear to be less value for money but in reality they are usually among the top value for money offerings!
A 650W SeaSonic unit is often a better choice than an 800W unit from a cheap and/or unknown brand. More expensive power supplies usually also have more extensive failsafe measures in place and are thus safer for the expensive components inside your PC.

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