Hardware Reference
In-Depth Information
The efficiency of any power supply varies with load. Efficiency is maxi-
mum at somewhere near the middle of its range. For example, a power
supply that is accurately rated for 400W maximum wattage will be most
efficient somewhere near 200W output, with efficiency gradually decreas-
ing as the load approaches either 0% or 100%. That's why it's a mistake to
buy too much power supply for your system. If your power supply is rou-
tinely operating at, say, 25% of rated wattage, the good news is that it'll
probably last forever, but the bad news is that it's operating inefficiently.
Conversely, it's a mistake to buy too little power supply. If your power sup-
ply routinely operates at 90% of rated capacity, it's also operating very
inefficiently and will probably die an early death.
Small differences in power supply efficiency may be significant if you're a
corporation with a fleet of thousands of desktop or server PCs. Otherwise,
give preference to the more efficient unit, but don't make it a deciding
factor.
Power-factor correction
Power supplies of a few years ago typically had power factors of 67% or so.
In essence, that means those power supplies drew 150% of their nominal
maximum input power two-thirds of the time and zero power one-third
of the time. For example, a power supply with a rated input of 300W and
operating at full capacity would actually draw 450W for two out of every
three cycles of the 60 Hz AC current, and 0W for the third cycle. In other
words, it would draw 450W for 1/30th of a second and then no power for
the following 1/60th of a second. That may seem a small difference from
drawing 300W continuously, but it has major implications for businesses
that must size their computer rooms' electrical infrastructure and UPSs for
the maximum current draw rather than an average draw.
Some years ago, power supplies with power-factor correction (PFC) start-
ed becoming more common. Most of these early PFC power supplies
used passive PFC, which amounts to a simple coil that helps smooth out
current draw. These units were able to improve power factors to 75% or
more. A few current PFC power supplies use passive PFC, but most use ac-
tive PFC, which uses live electronic components to smooth current draw.
A typical active PFC power supply improves the power factor to 90% or
more, and the best reach 99%. For home use, PFC efficiency is a minor
issue, but it's still worth choosing a power supply with a high PFC factor,
if only because such power supplies are generally better-made than units
with lower PFC factors.
Use the following guidelines to choose a power supply appropriate for your
system:
• Above all, make sure the power supply you buy fits your case and has
the proper connectors for your motherboard and, if applicable, your video
card(s).
• Assuming honest wattage ratings (like those from the vendors we recom-
mend), for a typical entry-level system, install a 300W or larger power sup-
ply. For a mainstream system, install a 400W or larger power supply. For a
high-performance system, install a 500W or larger power supply (possibly
