The efficiency of a computer power supply is its output power divided by its input power; the remaining input power is converted to heat as expected under
conservation of energy. For instance, a 600 W power supply with 60% efficiency running at full load would draw 1000 W from the mains and waste 400 W as heat. {{Stacked bar A 600 W power supply with 80% efficiency running at full load would draw 750 W from the mains and waste only 150 W as heat. {{Stacked bar {{Stacked bar For a given power supply, efficiency varies depending on how much power is being delivered. Supplies are typically most efficient at between half and three-quarters load, much less efficient at low load, and somewhat less efficient at maximum load. Older
ATX power supplies were typically 60% to 75% efficient. To qualify for 80 Plus, a power supply must achieve at least 80% efficiency at three specified loads (20%, 50% and 100% of maximum rated power). However, 80 Plus supplies may still be less than 80% efficient at lower loads. For instance, an 80 Plus, 520 watt supply could still be 70% or less efficient at 60 watts (a typical idle power for a
desktop computer). Thus it is still important to select a supply with capacity appropriate to the device being powered. It is easier to achieve the higher efficiency levels for higher wattage supplies, so gold and platinum supplies may be less available in consumer-level supplies of reasonable capacity for typical desktop machines. Typical computer power supplies may have
power factors as low as 0.5 to 0.6. The higher power factor reduces the peak current draw, reducing load on the circuit or on an
uninterruptible power supply. Reducing the heat output of the computer helps
reduce noise, since fans do not have to spin as fast to cool the computer. Reduced heat and resulting in lower cooling demands may increase computer reliability. == Shortcomings ==