Usually when there’s talk about computer power supplies, the main concern is one of the following:

• low price
• quiet (low noise)
• sufficient wattage
• stable voltage on the +3.3 +5 and +12 volt rails

Rarely is the efficiency a big concern. However, it should be.

Efficiency is a measure of how many watts of incoming AC power the unit takes to obtain a certain amount of DC watts to the various computer components. Unfortunately, PSU’s are not 100% efficient. That means delivering 300 Watts to your computer will be sucking more than 300 Watts from the power outlet. Simple math can help you determine how much it actually takes. For example, a PSU with 50% efficiency will take 600 Watts of AC power to deliver 300 Watts of DC power. One with 75% efficiency will only take 400 Watts of AC power to deliver that 300 Watts.

So why is this a concern? A couple of reasons….

Heat – For those who didn’t love physics when they were in school, you might be wondering… if a PSU takes in 400 Watts but only delivers 300 Watts to the computer, where did that extra 100 Watts go? The answer is it turned into heat. The less efficient a PSU is, the more heat it generates. Heat is bad for computers. Fortunately, the fan in the power supply blows most of the heat out to keep it cool, but then you’re heating the room. If it’s the middle of winter and your room is cold, you might not mind, but in the middle of summer, most people are spending time trying to keep their rooms cool, and a PSU pumping heat into your room is never a good thing.

Electricity – Think power bills. That wasted wattage that is generating heat is also racking up your power bill. It’s electricity that’s not helping your computer go faster, it’s just generating heat. Not only that, but if you’re air conditioning the room too, it’s costing you even more in power, because now the air conditioner also has to work harder to remove the heat the computer’s power supply is generating. Terrible stuff really.

The good news is that by using a high-efficiency power supply, you can ensure that you’re getting the most bang for your buck electricity-wise. You can also keep your room from feeling like a furnace.

Next time you look to buy a power supply, check the efficiency ratings. Yes, sometimes the manufacturer exaggerates, but if you look for a well known brand name such as Antec, Thermaltake, Vantec, Fortron, or Enermax, the efficiency rating should be accurate. Try to find a power supply with an efficiency of at least 70%. Note that most power supplies are more efficient towards the higher power ranges – one might achieve a rating of 70% at 200 watts, and 74% at 400 watts.

Tom’s Hardware has done a number of power supply stress tests which you may find interesting. They also do some pretty significant technical power supply tests:
Tom’s Hardware Power Supply Stress Test
Tom’s Hardware Power Supply Stress Test 2
Tom’s Hardware Power Supply Stress Test 3