Power supply units and efficiencyMay 22nd, 2011
Computer Power Supply Units or PSUs convert mains Alternating Current (AC) to the Direct Current (DC) required by computer parts. Power supplies are typically rated in watts of power they can reliably supply to a computer. But these PSUs aren’t 100% efficient at their job. Some power is lost during the conversion from AC to DC. How efficient they are at their job determines how much electricity your computer uses and that is reflected in your monthly electricity bill.
A typical generic PSU will have an efficiency rating of 60%. That is 40% of the power it consumes will be wasted.
There is a certification system for branded power supply units called 80+. This requires power supply units to be at least 80% efficient. There are different levels of 80+ certifications. They go from just 80+ standard to bronze, silver, gold and platinum. The higher levels are for more efficient power supply units with 80+ platinum being 90% efficient.
Lets take a look at how a typical generic 500W power supply unit compares to a 80% efficient branded one when operating at full load. That is when it is supplying 500W of power to the computer system:
60% efficient PSU = 500/60 x 100= 833W
80% efficient PSU = 500/80 x 100= 625W
So by using a more efficient PSU you save about 200W per hour or 1600W (1.6KW) per day if your system runs 8 hours a day at full load. Electricity prices are measured in units of 1000W consumed in an hour or 1KW. At the time of writing each unit of electricity costs between Rs. 7-10 in Pakistan. So you can now calculate your savings from using a more efficient PSU.
Of course in reality few computers use 500W all the time. Most modern computer system units with integrated graphics idle at 40-60W and use 80-120W under moderate load. So you also have too look at how efficient these PSUs are at supplying less than the rated power output. That is how efficient are they at supplying 20% or 40% of the rated power wattage? Can they also achieve 80% efficiency at that load level? The beauty of the 80+ efficiency rating is that it requires power supplies to be 80% efficient at 20%, 50% and 100% of the rated load.
But when your computer is consuming little electricity in the first place the savings to be had from a more efficient PSU are much less. So little in fact that the monetary savings don’t make them worth the extra expense in the short term. In the long term the efficient PSUs may save you money depending on how much power your computer consumes, how many hours a day you use it and the price of electricity.
Now what happens to the power that is wasted during the conversion from AC to DC? That power is wasted in the form of heat given out by the PSU. The less efficient the PSU, the more heat it generates. In the very hot Pakistani weather you might want to buy an efficient power supply unit just to help keep your room cool!