PcInside

Introduction

The power supply is the connection between your computer and the power grid. The power supply converts the 110-220V to a usable voltage (3.3V - 12V). On the backside of the powersupply you will find the cable that goes into the power grid. On the inside of the pc several cables are providing power to multiple components. Some cables provide power to the motherboard (including the memory, cpu and other connected cards) others make sure the DVDburner, Grapgics card and hard drives are funtioning. For a power supply it is important that it has enough avaible Watt and Ampere to deliver the required power. When the pwoer supply is too weak (this can occur when you upgrade your graphics card) you can get an error from the operating system explaning this power shortage or the entire system will just shut down. In worst case scenario you will see a blue flash and your power supply has "died".

Types of power supplies
Power supply can be distinguised on serveral levels; the wattage, de efficiency, het type of plug/connector and the amount of Ampere

-Wattage and Ampere
A power supply is differentiated on the basis of it's Watt, however a far more important factor to recon with is Ampere. If a power supply has a lot of watt this does not automatically mean its quality is high. Most of the time, especially for cheaper power supplies, it is hard to find the amount of Ampere avaiable. Before we explain this it is important to know how  power supply operates. A power supply has several "rails" each rail has it's own voltage because different components require different ammounts of power. The most important rails are: +12V, -12V, +5V, -5V, +3,3V and -3,3V. A high quality power supply has several rails for the most demanding cables (e.g 12v).

So what is this Ampere? The ampere shows the "size" of your electricity wheres watt is an expression of energy. Although this can be quite complicated to comprehend you do not need a physics degree before you can buy your own power supply. All you have to know is that the ammount of power a power supply can deliver depends on the combined sum of the avaiable ampere from each rail, companies convert these figures into watt. Because in the past the 3.3 V and 5V rails were important these can have a high ampere value. However nowadays the 12V rails has become more important, however these are also more expensive to manufacture. So when buying a cheap power supply unit with a high Wattage a common explanation for its low price (and perceived bad quality) is that it has a lot of ampere on it's 3.3V and 5V line but not on the 12V line. Example: A 30 euro 400watt PSU could have a lot of power on it's 3.3v thereby reaching the "400W" , when installing a graphics card problems arise. The replacement is a 50 euro 350W PSU, this one has a better division and is therefore better equipped to supply power to where it's needed.

-Efficiency
The efficiency of the power supply (psu) is the percentage of power that is converted into something  usefull for the computer. Example: a power supply is 85% efficient and is rated 300W, this power supply then will draw 352W from the powergrid (100%). A power supply that is less efficient will have a higher value; a 75% efficient 300W power supply will draw 400W from the grid. Point of attention: A power supply unit is most efficient around 50% usage. So when your pc normally uses 250W it is advisable to buy a larger 450W or 500w power supply. A 350W power supply will do the job but less efficient and will this result in a higher electricity bill. In the graph below you can see that the optimum levels lie between 40% and 60% usage. A rule of thumb is to double the amount of power your pc will use. E.g. 350 watt-->700 watt power supply.

De load of the power supply, the used capacity also influences sound levels. More usage results in more heat and makes the fans spin harder and thus louder.

Example: untill 60% the PSU is relatively silent, after this point the rpm almost doubles, from 1000 rpm to 1800rpm.

-Type of connectors/plugs

At this point in time 2 connectors/plugs exist:

• ATX
  De "old" standard
• ATX 2.03, ATX12V 2.0 
  De latest standard

Both belong to the ATX standard, so both can be used within an ATX case (in the future it is expected for BTX to gain popularity, however these cases are rare at this point in time).

If you PSU does not fit it is possible to buy a connector to bridge the different plugs, you could also try to return the power supply to the store, the store can also provide these bridging plugs.

How much watt does my pc require/use?
-theoretical calculation
Not a lot of people know how much power their pc's use, let alone how much power their future pc will use. To sovle this several websites exist that calculate the expected power usage according to the components you havee installed into your system:

• http://extreme.outervision.com/psucalculatorlite.jsp
• http://www.journeysystems.com/?power_supply_calculator

-recommended power supply, look at the graphics kaart
If the previous option requires too much information then there is a simpler solution. When you buy a pc with a decent graphical card (80 euro and above), then you can simply look at the system requirements of this component. You can either find this on the box/manual of the graphics card or at the website of the specific manufactorer. AMD Ati even supplies a list with approved power supplies from a whole range of brands.

-Rule of thumb:
Do you not have an expensive graphics card then 350W will be suffice
If you do have a dedicated graphics card then 450W will be suffice
If your system has two or more graphics cards then 750W or more is advisable (check http://www.slizone.com/ for Nvidia certified power supplies or , http://ati.amd.com/certifiedPSU/index.html  for Amd Ati power supplies).

Power consumption in practice
PcInside did some benchmarks, these are small scaled, it's just an example. Power usage was measured outside of the pc so it's already adjusted for power supply efficiency:

game1: Intel Core2Duo E6750 2.66 Ghz @3.2 Ghz, 4x 1GB DDR2, 3x Seagate 7200.11 500GB, Asus p5n32-eSLI, Nvidia Geforce 8800GTS, DVDbrander, DVDspeler, fancontroller Zalman MFC2, 7x 12 CM fan, 1x 80mm Fan
game2: Intel Pentium 4 2.54 Ghz, 2x 1GB DDR2, 2x IBM 80 GB, 1x Seagate 7200.9 320GB, Nvidia Geforce 6800GT, DVDbrander, dvdspeler, CDrw brander, 4x 80mm Fan
budget1: Dell GX240

Sources:

Power supply graphs: thermaltak thoughpower 750W http://nl.thermaltake.eu/product_info.aspx?PARENT_CID=C_00000900&id=C_00000974&name=Toughpower+750W&ov=n&ovid=