Why you should consider it: How much power a computer requires is a complicated question that everyone wants a simple answer to. Five years ago the simple answer would have been "anything over 250 watts". Today's answer may more appropriately be "300 watts is minimum while 350 is safer but 400 watts would be an even better choice". The simple answer changes as technology changes but one rule of thumb that hasn't changed is not to "cheap out" on the power supply.

When was the last time you looked at a power supply in the computer case? Have you recently bought a power supply smug in the knowledge that "you've bought the best"? What if that expensive 400 Watt power supply...wasn't? What's on the label isn't necessarily what comes out the cable. Take a closer look with us at power supply specifications and find out if you've really got the power.

The PSU (Power Supply Unit), with its wires dangling like the legs of an octopus that has been nailed to the inside roof of the pc case, either came with the PC or it was an aftermarket choice after researching a forum or review. How many of us really know what the power supply specifications mean, if specifications are distorted and how that could affect the operational stability of a PC?

A power supply isn't a top of mind concern for most. The computer turns on, it works, it turns off. There's no problem until there is a problem. Solution? Pick up the phone and call that 24/7 support line. This is, of course, if you knew that the power supply was the problem to begin with. Then there are those who bought a PC from an OEM retailer to save a few bucks. The PC is packed up and ferried down to the store and it is taken away and the power supply is replaced with the same generic version or its cheap alternative that is presently in stock.

Problem solved?

Go back to the symptoms. If the PC just shut down or wouldn't power up and there was an acrid smell in the air then the power supply most likely burnt out. If the PC is unstable where it suddenly reboots or displays that pretty Blue Screen of Death then the power supply may not be "up to the job". These instabilities are a common complaint. They often occur after upgrading a processor, video card or adding another drive. If there is a stunned silence and eyes are left blinking with the hanging question "how am I supposed to know?" then the most simple solution is to upgrade to a beefier power supply to handle the extra load. For overclockers and PC enthusiasts it isn't as simple as the decision to upgrade but what power supply to upgrade to? In either case even a beefier power supply may still be the root of system instability. Wattage isn't everything and there is a way to make informed choices.

So how much power do you need?

How much power a computer requires is a complicated question that everyone wants a simple answer to. Five years ago the simple answer would have been "anything over 250 watts". Today's answer may more appropriately be "300 watts is minimum while 350 is safer but 400 watts would be an even better choice". The simple answer changes as technology changes but one rule of thumb that hasn't changed is not to "cheap out" on the power supply. Cheap, generic, bargain power supplies only lead to system instability. The "bargain" power supply may be inadequate to supply the actual power that a computer draws at certain voltages and/or amperages.

A few basic ground rules need to be set before getting "up close and personal" with a power supply.

Zzzzzzzap!

First and foremost a power supply is an electrical device which, if not used correctly, has the ability to damage components, cause bodily harm or even death. Do not remove the cover or modify a power supply if not qualified to do so.

AC/DC

The power supply's job is to take AC power (Alternating Current) and convert it to DC (Direct Current) power. Unfortunately the computer power supply doesn't do a perfect job of conversion. In fact approximately one third of the AC power coming into the power supply is wasted in the conversion process. This "wasted" power takes the form of heat, which is exhausted by the power supply fan, and as a high-frequency signal or ElectroMagnetic Interference (EMI). EMI is one of the reasons why a power supply is contained within a metal casing. The metal casing primarily prevents curious fingers from touching live electrical components and thus possibly bbq'ing the owner of said fingers. Secondarily the metal casing is a shield preventing those high-frequency signals from freely bouncing about inside the PC case and possibly wreaking havoc with other electronic gizmos and doodads that rely upon their own high-frequency signals to operate properly. If the power supply were not shielded it would be like trying to listen to two radio stations at once. The result would be a mish-mash of interference making it impossible to make sense of either one.

Different Jolts for Different Folks

Power supplies also produce DC at different voltages to meet the requirements of the particular component. In the most common power supply style, ATX, these voltages have been been standardized.

Voltage	Common Use
-12	Used on some serial port circuits. Not needed on newer systems and if required on older systems is supplied at a very low amperage. <1A
-5	Was used by floppy controllers and ISA bus cards but now as common as a Pentium 90 processor.
+3.3	Used by newer processors, AGP video cards, system ram and other motherboard circuits
+5	Used on older motherboards to power components including the PSU. Manufacturers are putting more emphasis on +3.3 and migrating away from +5 voltages. +5 is still used to power floppy drives and optical drives.
+12	Used to power disk drive motors in devices such as hard drives and motors in optical drives.

Riding the Rails

Each of these voltage lines are also commonly referred to as "rails". A rail is defined as "either pole or conductor of a power supply, one is positive and one is negative." If someone asks "what's the five volt rail?" then they are referring to a voltage that a meter or software program would show in reference to the amount of voltage output on the designated five volt line.

Efficiency Rating

An interesting specification in a power supply is the efficiency rating. This rating takes the form of a percentage and is a measure of the power supply's ability to convert AC to DC. A power supply that has a 50% efficiency rating will only convert half of the AC input power into DC while the other 50% is byproduct produced in the form of heat and EMI. Higher efficiency numbers are better. The efficiency number will affect the power bill at the end of the month. A less efficient power supply can waste AC like my V8 truck goes through gas.

PFC is not Private First Class

The use of "PFC" in a power supply's description may be confused with the term "Power Factor". Explaining power factor isn't that easy but it is defined as the ratio of real power to apparent power.

Real Power: In alternating-current power transmission and distribution, the product of the rms voltage and amperage, i.e., the apparent power, multiplied by the power factor, i.e., the cosine of the phase angle between the voltage and the current. Note: Only effective power, i.e., the actual power delivered to or consumed by the load, is expressed in watts. Apparent power is properly expressed only in volt-amperes, never watts. Synonym true power.

Apparent power: In alternating-current power transmission and distribution, the product of the rms voltage and amperage. Note 1: When the applied voltage and the current are in phase with one another, the apparent power is equal to the effective power, i.e., the real power delivered to or consumed by the load. If the current lags or leads the applied voltage, the apparent power is greater than the effective power. Note 2: Only effective power, i.e., the real power delivered to or consumed by the load, is expressed in watts. Apparent power is properly expressed only in volt-amperes, never watts.

Source: Institute for Telecommunication Sciences. (ITS)

See what I mean? I "phased-out" too and it gets more complicated from there. PFC means Power Factor Corrector which is a converter device that precisely controls input current to match the waveshape of the input voltage. In layman's terms the PFC acts like a traffic cop. A power supply will do what is asked of it; some more efficiently than others. If the PC components ask for 100 watts of power then the power supply demands AC current from the wall and converts it to the 100 watts required. An inefficient power supply can ask for more AC than it needs. A PFC equipped power supply allegedly reduces this by being more precise when asking for the AC current to convert to DC.

The utopian goal is to derive the greatest amount of usable power from the least amount of input line current. A 100% efficient power supply would produce no heat or EMI and convert 100% of the AC input to DC power. A PFC equipped power supply may not affect the overall wattage performance but it allegedly makes the power supply more efficient at managing a stage of converting AC into DC and therefore reduce energy costs.

The Peak of Power

Power supplies can be rated with two wattage numbers; peak and continuous power. The peak power rating is usually in parenthesis right behind the continuous power rating number. During normal operation the power supply has been rated to be able to deliver a specific wattage all the time. This is the continuous power rating but the PC can be a power pig during startup. Components, such as hard drives, can chew up a lot of power when coming up to speed. Have you ever tried to push a car? It takes a lot of effort to start it rolling but it gets easier afterwards. Power supplies can have a "peak" power rating to accommodate for this spike. The power supply can sustain a higher wattage for a short period of time. This short period of time may be less than a minute but it is enough to "get the car rolling."

Now that the basics are out of the way coming to the definitive conclusion of how much power a PC uses requires a bit of detective work. The amount of power that a component draws fluctuates over time. Not all components will ever draw their peak and most certainly not every component will make this heavy demand on the power supply simultaneously.

Stop!

It is important to understand two concepts before progressing any further. First:

Watts = Volts x Amps

Power drawn by an electrical PC component is measured in watts which is an expression of the product of volts and amps. A processor that draws 50 watts can function at 25 AMPS on a 2 Volt line providing the manufacturer has engineered it to be able to do so. Second:

Combined Wattage Rating

Power supply lines can be rated independent of each other or combined. Typically ATX power supplies are rated with a maximum wattage for the +3.3 and +5 line combined meaning that the total of both, no matter the ratio, cannot exceed the designated rating. This is an extremely important number to note when looking at power supplies. A manufacturer may even combine the +3.3, +5 and +12 line and provide a maximum wattage rating but this is not as common.

Which more is better?

If wattage is the product of volts and amps then is it better to have more volts or is it better to have more amps? A simple analogy would be that voltage and amperage is like a dam across a river and water is electricity. As the water begins to build up against the dam the pressure increases. This is voltage. Now the dam opens a floodgate. The amount of water that is allowed through is amperage. The more the floodgate is opened the more amperage there is.

Electrical gurus everywhere will most likely debate this point with a lot of complicated math, talk of resistance loads and so forth. That's the problem with analogies. They are oversimplified but it serves the purpose well for basic understanding. More voltage isn't preferable to more amperage. More amperage isn't preferable to more voltage.

huh?

Yes it's time for those two nebulous words we all love to hear; "it depends". In the most basic form adding loads to a single line is a sum of wattages. 4 devices requiring a constant 20 watts each will draw 80 watts. As long as the line can deliver a minimum of 80 watts then there is no problem. It's always better to have a minimum of 1.5 times the wattage needed.

The biggest problem is if the sum of all wattages exceeds what the line can supply. For amperage look to the device that requires the highest amperage and not the sum of all amperages. The same again for voltage. On a 150 watt line 4 devices requiring a constant 40 watts would be a problem because a total of 160 watts is required; 10 more than is available. On that 150 watt line that can supply a maximum of 30 amps then if 3 of those devices required 20 amps each but one required 40 amps then the 40 amp device would be the problem. If any one of those devices on the 12 volt, 150 watt, 30 amp line required 15 volts there would be a problem again.

So the "it depends" depends on the load being put on the PSU on the specific line or lines. Motherboards have built in power converters to switch, for example, +3.3 and +5 volt power to a required CPU voltage such as 1.65 volts. As voltage goes down the maximum amps supported by that line will go up. Again...the product of volts and amps equals watts and as long as that math doesn't exceed the maximum rating for that line or the combined lines then all is relatively well.

Plug in the amps

Amperage is another tricky devil that plays funny tricks on a computer. Devices that operate at the same voltage input can operate on a line that has an amperage rating sufficient to the device that has the single highest amperage requirement.

Device A requires 10 amps. Device B requires 20 amps and Device C requires 5 amps. Each device takes only what it needs and amperage is available current but doesn't mean the excess is forced down the device's digital throat. Therefore a 20 amp line will support all three devices PROVIDING there is sufficient wattage. Remember that wattage is the sum of all totals of volts times amps. So if these devices were 12 volts then:

Device A = 12 volts x 10 amps = 120 watts

Device B = 12 volts x 20 amps = 240 watts

Device C = 12 volts x 5 amps = 60 watts

A single 20 amp line that had a minimum wattage rating of 420 watts would be needed. That's the pure physics answer. Amperage needs to be added but voltage does not for a series circuit on a single line and vice versa for parallel but wattage is a sum in both situations. So if wattage is fixed then voltage can be stepped down to increase amperage and vice versa but math is an unforgiving beast. More voltage and more amperage cannot be demanded simultaneously if it exceeds the maximum wattage rating.

The question probably remains "then how do I figure it all out?" The answer is to go to electrical engineering school because it's not that simple but the lesson in all of this is a simple number that often isn't on the power supply label that can separate a good choice from a bad choice.

Adding it all up

Adding up power requirements in a PC is a game of cat and mouse. Sometimes it isn't easy to find this information. Sometimes all of it isn't completely available. But if precise power is the goal then it's important to don your Sherlock Holmes hat and cloak and begin the hunt. In the following example the maximum demand during normal operation has been listed according to component. These numbers will, of course, change during standby, lesser activity or idle modes. These, but not all, numbers will also increase during overclocking. The following example shows an example PC's potential maximum power requirements by component during 100% usage which would rarely, if at all, occur.

	AMPS	Volts	Watts	Voltage lines
AMD 3200+ Processor	46.5	1.65	76.8	+3.3
ATI 9700 PRO ++++	67.5	0.8	54	+3.3/+5
Maxtor 7200 RPM ATA/133 80 GB HDD*	0.858/0.662	5/12	4.29/7.944	+5/+12
Liteon 52x CDRW	1.5/1.5	5/12	7.5/18	+5/+12
Mitsumi floppy drive			3.9	+5
ADDA 31.4 CFM 28.3 dBA 80x80x25mm**	0.15	12	1.8	+12
Motherboard			20	all
RAM 1 x 256 MB++	3.76	2.6	9.776	3.3
Chipset fan			1.5	+12
300mm Cold Cathode light	0.005	12***	3.4	+12
Round CCFL fan (light + fan)	0.135	12	3.96	+12
Audio (Creative Labs Audigy with 1394) +++	0.450	12	5.4	+12

• * Seek time measurement 5V/12V rating draw.
• ** 8 x fans inclusive of heatsink fan.
• *** Output voltage of inverter:680vrms.
• ++ OPERATING CURRENT: Four device bank interleaving READs (BL= 4) with auto
  precharge,tRC = minimum tRC allowed; tCK = tCK (MIN); Address and control
  inputs change only during Active, READ, or WRITE commands. 3,760 mA
• +++ Creative Labs Sound Blaster® Audigy™ Audio with IEEE 1394
  Power Consumption +12 V, nominal current 160 mA
  Power Consumption - Vcc, nominal current 450 mA (voltage controlled current source)
  Power Consumption -12 V, nominal current 50 mA
• ++++ in AGP 8x mode 0.8v. AGP 4x mode = 1.5v or 0.8v.
  

Categorizing the components by amperage, voltage or wattage on each of the lines may also reveal important information before choosing a power supply.

	3.3 Volt Line	5 Volt Line	12 Volt Line	3.3 Volt Line	5 Volt Line	12 Volt Line
	Amps	Amps	Amps	Watts	Watts	Watts
AMD 3200+ Processor (draws 1.65 Volts)	46.5			76.8
ATI 9700 PRO Video Card				53	1
Maxtor 7200 RPM ATA/133 80 GB HDD*		0.858	0.662		4.29	7.944
Liteon 52x CDRW		1.5	1.5		7.5	18
Mitsumi floppy drive					3.9
ADDA (2 rear, 1 blowhole, 2 front intake) 80x80x25mm			0.75			9
Motherboard						20
RAM 1 x 256 MB	3.76			1.8
Chipset fan						1.5
300mm Cold Cathode light			0.005			3.4
Round CCFL fan (light + fan x 2 side intake)			0.135			3.4
Audio (Creative Labs Audigy with 1394) +++			0.45			5.4
Total	46.5	1.5	1.5	131.6 W	16.69 W	68.644 W

The totals when compared to power supply specifications may show, for example, that peak wattage demands on the 12 volt line exceeds what the power supply has been rated for. In the preceding example the total required maximum wattage that could be required by the PC components is 216.934 Watts. Then why the need for these 300, 400 and 500 Watt PSU behemoths? It appears that a 250 Watt power supply could do just fine with a little under 35 Watts to spare.

Wattage isn't everything and a look at the PSU label may save hours of grief and frustration.

The astute may have noticed that the amperage totals are wrong. Remember that lesser amperages can exist on lines with devices requiring higher amperages as long as the wattage is sufficient for both. It is the device with the highest amperage that sets the proverbial pace.

Watt's on the label?

By now the head must be spinning with numbers and it seems that the truth about power supplies is no closer; but it is. A label can tell a lot about a power supply even if it doesn't tell a lot. The following are the specifications for an actual power supply rated at 420 Watts with a 520 Watt peak load. This is what typically is printed on the label. The name brand and model number is irrelevant for this example.

Volt line	+3.3	+5	+12	-12	-5	+5 Vsb
Amps	26	42	18	1	0.8	2.5

Remember wattage is volts times amps.

Volt line	+3.3	+5	+12	-12	-5	+5 Vsb
Amps	26	42	18	1	0.8	2.5
Watts	85.8	210	216	12	4	12.5

Contrary to what might be assumed, the rated power or wattage the power supply can provide is not the sum of all the wattages. Therefore:

85.8 + 210 + 216 + 12 + 4 + 12.5 does not equal 540.3 Watts.

This is because each line is not independent of the other for the maximum rated output. Once the power supply is tested on multiple lines simultaneously the maximum wattage is reduced accordingly. The maximum individual rated wattages on the label can only be attained if ONLY the single line were drawn upon. The full reasoning involves a lot of math and quoting of such things like OHMS LAW...take my word for it. 540 Watts is a bit over the 420 Watt continuous power specification. Wattages for the individual rails are indeed possible if only that line were used but the PC doesn't work that way.

Manufacturers supply combined ratings in addition to the maximum rated output. A combined rating is the maximum wattage possible of two or more lines drawn upon, each in any amount, simultaneously. This will be either the combined +3.3 and +5 volt line rating or the combined +3.3, +5 and +12 volt line rating. It means the maximum wattage that can be supplied if those two or three lines were drawn upon simultaneously.

Specification hocus pocus: the good

This is where what isn't on the label is as important as what is. A manufacturer should supply the combined rating for the +3.3 and + 5.5 volt line and then supply the combined rating for the +3.3, +5 and +12 volt line.

Volt line	+3.3	+5	+12	-12	-5	+5 Vsb
Amps	26	42	18	1	0.8	2.5
Watts	85.8	210	216	12	4	12.5
combined	220
combined	400

This shows that if the maximum is asked of the +3.3 and +5 volt line simultaneously then the power supply will deliver a maximum of 220 watts on those lines. Remember that the demand does not have to be equal on each line. It can be any amount as long as the total wattage equals 220 watts. For example if a 15 amp draw is put on the +3.3 line that is a total of 49.5 watts therefore 170.5 watts remains available on the +5 line which would be a maximum of 34.1 amps. (5 volts x 34.1 amps = 170.5 watts)

It also shows that if the maximum were asked of the +3.3, +5 and +12 volt lines then the power supply will deliver a maximum of 400 watts. For purpose of simplified explanation the same rule of balance applies.

The remaining 20 watts of the 420 watt continuous power rating comes from the addition of the remaining lines which is 28.5 watts. The manufacturer rounds it down to 420 watts most likely because a 428.5 watt power supply is an odd rating to market. It leaves a little margin for error too. This is good information on a label.

Specification Hocus Pocus: The better

A really handy specification that rarely is included in power supply specifications is the maximum amperage of the combined lines.

Volt line	+3.3	+5	+12	-12	-5	+5 Vsb
Amps	26	42	18	1	0.8	2.5
Watts	85.8	210	216	12	4	12.5
combined	220@30 amps
combined	400@15 amps

Math comes to the rescue again. A power supply could supply 400 watts of power BUT if it is 1 volt and 400 amps then it quickly changes the picture for a 12 volt device. If a device that requires 30 amps draws upon the combined +3.3/+5/+12 line which is 12 volts then

12 volts x 30 amps = 360 watts

Nearly all of the power supply's resources have been eaten up by a single device. This is an extreme scenario but when all the components in a real system are added up and amperages taken into account the power supply could be less than adequate for the job. In a nutshell it is the amperage that limits the overall capability. The simple rule of thumb is that more amperage on the specific line or combined line is better. It is disappointing that manufacturer's don't readily release this information as it can have a profound effect. Two different power supplies may both be 400 watts but the if the amperage on the combined lines is less for one then that PSU may greatly affect system stability.

Specification hocus pocus: the not so good

This power supply manufacturer rates the following example at 460 watts but notice what is missing.

Volt line	+3.3	+5	+12	-12	-5	+5 Vsb
Amps	35	35	33	1	1	2.2
Watts	115.5	175	396	12	5	11
combined	185
combined

The +3.3 and +5 volt combined rating is supplied but not the +3.3, +5 and +12 volt line combined rating. It cannot safely be assumed that the combined +3.3, +5 and +12 volt line is approximately 430 watts (460 watt rated power minus the sum of -12, -5 and +5 VSB line). It cannot be assumed that the manufacturer is trying to hide a defect. The problem is the information isn't available to know.

This is where it is important to look carefully at power supply specifications. The maximum rated output is known which is 420 watts. This number is usually very close (within only a few watts) of the sum of the combined +3.3/+5/+12 volt rating and the remaining negative lines therefore some simple math:

420 - (12 + 5 + 11) = 392

392 watts is only an assumed approximation due to missing information but it is valuable to know. This will become apparent when comparing known values of the combined +3.3/+5/+12 rating in other power supplies of similar maximum wattages. Further on in this article it will be shown that the average is around 380-410 for this range which makes the predicted 392 watts believable. Remember it isn't verified by the manufacturer so use caution.

What IS important is comparing the individual wattages to the combined totals....either the combined +3.3/+5 rating or the combined +3.3/+5/+12 rating. The prediction is for a 392 watt rating on the combined +3.3/+5/+12 volt line which is less than the individual +12 volt line maximum wattage. Individual maximum wattages can NEVER exceed the combined totals they are governed by.

This predicted combined +3.3/+5/+12 volt rating and the associated individual 12 volt rating are now suspect. This power supply should be looked upon with skepticism. THAT is the story that specifications can tell. Be cautious of figures like those that are too close or over the combined ratings.

We got graph: 23 power supplies compared

The test is that of specifications. 23 power supplies have been grouped into a 350-360 watt range and 400-470 watt range. All specifications are according to the manufacturer. Null values indicate that specifications were not available on the manufacturer's website. Pricing was determined by lowest price on Pricewatch.com the date of this article except Channel Well Technology (approximated by nearest model), Topower (n/a) & PC Power & Cooling (from pcpowerandcooling.com).

Each group is sorted by:

• +3.3/+5 volt line combined wattage rating
• +3.3/+5/+12 volt line combined wattage rating
• +3.3/+5 volt line combined wattage rating as a percentage of maximum rated output
• +3.3/+5/+12 volt line combined wattage rating as a percentage of maximum rated output
• price
• maximum rated output

350-360 Watt Power Supplies

 

 

 

 

 

 

 

350-360 Watt conclusions

Since 6 of the 8 power supplies were 350 watts and the remaining two were each 360 watts the playing field is fairly even. With the available data only the Coolmax CX350 fell slightly behind the pack (a mere 4.5 watts) for the maximum output of all positive lines (+3.3/+5/+12).

The +3.3 and +5 combined lines are the driving force behind the DC power requirements for GPU and CPU and therefore of greater interest. Surprisingly, considering Enermax leads the pack as one of the most expensive, it takes last place delivering 185 watts of combined +3.3/+5 DC power.

Antec is the leader turning in 230 watts of +3.3/+5 volt combined. The Antec SL350 is more expensive than the pack average but not the most expensive. Enhance challenges with an equal +3.3/+5 combined wattage rating at a few dollars less. The downfall is that enhance does not supply a combined +3.3/+5/+12 rating. The Enhance ATX-1136H is rated at 360 watts which means the combined +3.3/+5/+12 rating should be, by extrapolation, approximately 340 watts. This is an unverifiable figure but if true it puts Enhance in a very good position.

400-470 Watt Power Supplies

 

 

 

 

 

 

 

400-470 Watt conclusions

Enermax continues to disappoint delivering the least amount of combined +3.3/+5 wattage compared to its higher than average price. This is where specifications can work for the consumer as it doesn't always show that "you get what you pay for."

PC Power and Cooling turns in the most expensive price tag but with that a whopping 300 watts on the combined +3.3/+5 volt lines. There is plenty of "oomph" there but only at a maximum output of 400 watts. Zalman turns in a respectable 235 watts on the +3.3/+5 volt lines and ranks above the price average but well below the two most expensive power supplies. It caught our eye due to Zalman's quiet CPU design. Similarly the Vantec 470 costs only a few dollars more but packs 70 more watts on the maximum rated wattage while still competing well with 235 watts on the combined +3.3/+5 lines. Our vote will go to the Vantec 470.

Truth and consequences

In a perfect world there would be a comprehensive database of controlled tests. Each power supply would undergo the same battery of laboratory tests. Unfortunately this does not exist and the consumer is faced with choice by price, by popular recommendation or using a bit of synaptic power.

This test of specifications illustrated what you, the reader, will have hopefully learned through reading this article. Specifications have to be looked at carefully in order to narrow down choices. Once those choices have been made then a purchase can be made or the search for further information is made that much easier.

There are some conclusions that can be made:

1. A higher efficiency rating percentage number will contribute to a smaller energy bill.
2. A PFC equipped power supply allegedly contributes to a smaller energy bill.
3. Any one PC component will rarely, if ever, reach its maximum load potential and it is a more remote chance that every PC component will reach its maximum load potential simultaneously.
4. Wattage on any given line or combined line should be a minimum of 1.5 times the required maximum. More if further expansion is planned. Remember that combined line wattage rating overrules the individual ratings they are combining.
5. More amperage on a given line or combined line is better. Same rule of combination as point 4.
6. More wattage on a given line or combined line is better. Same rule of combination as point 4.
7. Be careful of the difference between the +3.3, +5 and +12 combined rating and the power supply rated output. It may say 400 watts but only deliver 350 on the positive lines where it is needed.
8. Be suspect of specifications that only supply the +3.3 and +5 volt combined rating and not the +3.3, +5 and +12 volt combined rating.
9. For choosing minimal power supply wattage for normal operation, as a loose rule of thumb, the sum of all wattages of all the PC components should be multiplied by 1.5 and rounded to the next available power supply rated output.

Power supply tests should be done under laboratory conditions. They should be carried out with professional testing equipment neither we, nor any other hardware review site, can or wants to afford. Professional DC load testers and associated equipment costs thousands and even tens of thousands of dollars. Short-Media's real world power supply tests and roundups will feature watt-hour comparisons and simulated load tests with voltage observations made on each of the positive lines. It may not be laboratory conditions but it will help in the decision making process.

The simple answer

Shocking isn't it? Not every power supply is what it appears to be. Specifications can and are distorted to provide more shelf appeal. Not every manufacturer is sleight of hand with their specifications and not every model is a sheep in wolf's clothing. So is there a simple answer? No but you want one regardless. At the time of this article we would say that a power supply with a minimum of 350 watts would be acceptable but we sure would recommend 400 watts.

But...

Overclockers and enthusiasts everywhere want to squeeze every bit out of their PC. Look to what setup you have; the "it depends" scenario. Planning to crank up the voltage on the processor and ram? That 3.3 and 5 volt line better have sufficient continuous wattage. Looking to RAID four drives together then a power supply with a higher amperage on the 12 volt line is preferable and make sure that line has a lot of wattage headroom. Component startup can sometimes double the demand on the power supply for a short period of time.

For overclockers and enthusiasts the answer isn't impossible to find. It just takes a bit more investigation to get there.