Efficiency is traditionally an overlooked power supply specification. It says how much power is being wasted while you are using your PC. Problem is that you are paying for this wasted power. The 80 Plus certification was created to allow consumers to know which power supplies are the most efficient ones and, as the name implies, guarantees that the power supply is able to present efficiency of 80%. With more and more manufacturers trying to design power supplies with efficiency well above 80%, they decided to create three new certification categories: Gold, Silver and Bronze. Learn what they really mean.
First let’s give you some background on efficiency. Efficiency, which is also represented by the Greek letter Eta (η), is given by dividing the power being delivered to the equipment (output power, i.e., DC power) by the power being consumed by the power supply (input power, i.e., AC power).
If you have a PC that consumes 250 W and a power supply with 75% efficiency, this means that you are pulling (and paying for) 333 W from the wall. If the same computer had a power supply with 85% efficiency, it would be pulling 294 W from the wall and you would be saving 39 W. Thus a power supply with higher efficiency will allow you to save money on your electricity bill.
Power supplies do not present a constant efficiency. The graph for efficiency is a bell curve, where the power supply presents its best efficiency when delivering 50% from its labeled load. We have an example in Figure 1.
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Figure 1: Example of an efficiency curve.
Because of this effect it is recommended that you buy a power supply with double the power you are actually going to pull. This explains the offer of high-wattage power supplies above 700 W. Manufacturers don’t expect you to pull the full power from their units, but that you operate them around 50% load for a higher efficiency (during our reviews, however, we need to see if the power supply can really deliver its labeled power, because if a power supply is labeled as, let’s say, 600 W unit, we want to be capable of pulling 600 W from it, if we want to). The only disadvantage to this approach is the price of a higher wattage unit. But at the long run it is a good idea, as you will save money on your electricity bill, you computer will run cooler, you have enough headroom for a future upgrade and you won’t face any stability problems when playing games at their maximum quality for hours.
The second thing you need to know about efficiency is that power supplies present a higher efficiency when connected to a 230 V (“220 V”) power grid, and the efficiency numbers announced by manufacturers are measured at this voltage. Thus if you live in a country or region where the power grid is 115 V (“110 V”) – like the United States – your power supply is likely to present an efficiency lower than the number announced by the manufacturer. Our power grid is 115 V and in our reviews we test power supplies under this voltage.