There are huge numbers of myths about Power-Supply and its features. Every single week, new models of Power-Supply come out with features never heard-off but also eye-catching at the same time. Mind bobbling wattage value, many rails, modular, SLI/CrossFire certified etc. But whats the real fact within them ?

Before I start, do remember, Power-Supply is equally important in choosing the right Processor+Motherboard combination. So, choose wisely.

Does a Power-Supply upgrade means a Higher Electric Bill ?

This is the biggest myth of all.
First, it's important to understand that a power supply only delivers the power that's needed by the system, nothing more. If your PC currently has a 400W supply and the system needs 350W, it will still need and use only 350W - if the only change is upgrading to a 500W power supply (the upgrade makes sense since there are many advantages to running a power supply at a lower percentage of its rated capacity). Whether the electric bill goes up or down is solely determined by the efficiency of the new power supply. Greater efficiency means a lower electric bill because more of the AC power is converted into DC for the computer, rather than wasted as heat. The savings can really add up over time.

Don't be misled by Exaggerated Wattage claims.

To properly compare power supplies, wattage claims must state the maximum ambient temperature for continuous, full-load operation. Unfortunately for the consumer, this information is usually withheld, opening the door for manufacturers to exaggerate their wattage claims. They do so by assuming an unrealistic ambient temp of only 25°C (77°F), even though the actual internal power supply temp is at least 40°C (104°F). Since the proper full-load rating is 15°C higher for home use and 25°C higher for industrial use, these power supplies produce 33%-50% less power than their advertised ratings.

Also some brands overwrite the wattage value than the value mention by the very own OEM company that actually builds the SMPS/PSU for them !!

Example: Truth about: Cooler Master eXtreme Power 600W  

Don't loose Power with Modular Plugs.

Due to their look, convenience, cost savings for manufacturers and for lesser cable management inside a PC, modular plugs have become a popular power supply feature. Unfortunately, there has been little or no discussion of the impact of this feature on overall performance and reliability. The fact is, modular plugs limit power by adding to electrical resistance. The voltage drop can be as much as would occur in 2 feet of standard wire. Worse yet, modular plugs utilize delicate pins that can easily loosen, corrode, and burn, creating the potential for a major system failure. That's why professional system builders specify uninterrupted wire !

An SLI-Certified Power-Supply will always power High-End Graphics Cards ?

We went through three power supplies before we found one that consistently worked with these high-end graphics cards and an FX-60. We initially started with a Silverstone 600W SLI certified power supply, but running some games resulted in the system powering down under full load. We next moved onto a higher rated Thermaltake PurePower SLI certified 680W unit. But while most games ran most of the time, the system would still occasionally shut down.

ExtremeTech Review Feb 12, 2006  

Are Multiple 12-volt Rails Better then a Single 12-volt Rail ?

With all the hype about multiple 12-volt rails (ads claim that two rails is better than one, five is better than four, etc.), you'd think it was a better design. Unfortunately, it's not !

Here are the facts: A large, single 12-volt rail (without a 240VA limit) can transfer 100% of the 12-volt output from the PSU to the computer, while a multi-rail 12-volt design has distribution losses of up to 30% of the power supply's rating. Those losses occur because power literally gets "trapped" on under-utilized rails. For example, if the 12-volt rail that powers the CPU is rated for 17 amps and the CPU only uses 7A, the remaining 10A is unusable, since it is isolated from the rest of the system.

Since the maximum current from any one 12-volt rail of a multiple-rail PSU is limited to 20 amps (240VA / 12 volts = 20 amps), PCs with high-performance components that draw over 20 amps from the same rail are subject to over-current shutdowns. With power requirements for multiple processors and graphics cards continuing to grow, the multiple-rail design, with its 240VA limit per rail, is basically obsolete.

Should an ATX Power-Supply be Cooled with a 120mm FAN ?

Most low-noise ATX power supplies today utilize a top-mounted 120mm fan rather than a rear-mounted 80mm fan. The 120's favorable reputation is based on the fact that under low to medium load conditions, the 120mm fan provides sufficient cooling at low RPM and low RPM fans are generally very quiet.

However, problems occur with this design when the load exceeds 50%-60%. Because the 120mm fan consumes about 1.5" of vertical space inside the PSU, heat sinks, capacitors, and other components are about 30% smaller in height compared to a PSU with a rear-mounted fan. The smaller parts can handle less current, so the maximum power available with the 120mm design is limited. And, because the heat sinks have less surface area, more air flow is needed with this design to keep the thermal situation under control. With 80%-100% load, the 120's fan speed can double and the noise level can jump by up to 20dB.

In conclusion, for systems that require more than 50% of the power supply's capacity, a well-engineered PSU with a rear-mounted 80mm fan will provide superior performance and reliability (due to larger components) at a noise level comparable to a PSU equipped with a 120mm fan.

Are Two Power-Supply Fans Better than One ?

No. A power supply with two fans doesn't exhaust any more air from the case than a power supply with one. That's because due to space limitations, only one fan can be used for exhaust, while the other is limited to spot cooling. Possible explanations for a 2-fan PSU include: thermal engineering problems; an attempt to exaggerate the wattage rating by spot cooling the transformer; or an attempt by the marketing department to create a new gimmick.  

Hope this article will help you in choosing the right Power-Supply for your need.