Don’t forget the power
So, you’ve finally decided which CPU, motherboard, and graphics card you want to put into your new computer. But what
are you going to do about the power supply? The power supply has always been the most esoteric part of system
building. We all have a vague idea that a power supply with more watts is better than one with fewer watts. We also
know, and expect different manufacturers to have different interpretations of “peak output.” In other words, you already
know that all 300W power supplies aren’t created equal -- you also know that 550W power supplies are overkill for a
standard desktop machine.
But how much power do you really need, exactly? Most of you probably don’t know the answer to this. In fact, before last month, I
didn’t know the correct answer either. Early on, I used to rely on AMD’s power supply list, but AMD stopped updating that. Since
then, I’ve just dropped back to my old-school criteria: start with a case/power supply combo that’s in the range of $50, and then put
down extra cash for a better design and cooling.
Why Power Supplies are Important
Everyone keeps talking about watts, and it’s an understandable approach. People always talk about high-end power
supplies being one reason why servers and workstations are more reliable than standard desktop machines. For
example, SGI Octane workstations have 800W power supplies, even though the highly efficient MIPS architecture CPUs
consume only about 17W each, the same as an x86 notebook chip.
But this is the wrong way to pick out your power supply. You first need to decide what the rest of your system will look
like, and figure out how much power that takes. Then you can buy a power supply that meets your budget. Sounds
simple, but there’s one little secret…
Budgeting your power needs
First things first: you need to think of your power supply not as a single unit, but a box that contains multiple, independent
power supplies. That is, if you look at the label on a power supply, you’ll see that the maximum current is listed
independently for +3.3V, +5V, and +12V. Thought of another way, imagine the total wattage being the power of the pump
at the water reservoir, and that there is a pump for +3.3V water, +5.5 water, and +12V water. Just be patient with me, it’ll
all make sense by the end of this article.
Determining your Power Budget
Some people tell you to add up the number of watts that your components support. For example, an AMD Athlon XP
2000+ (Palomino) uses 87.5W. Your motherboard is 23.5W, and so your power needs are 111W for the moment. Add up
all your components and you’re done. The problem is that because the power supply is not a unified source of energy,
the 87.5W from the Athlon XP is meaningless unless you know how those 87.5 watts are distributed; are they on the +5V
rail or +12V rail?
And therein lies the problem. You see, the Pentium III and all CPUs before it ran on the +5V rail. Since the CPU remains
the largest consumer of electricity in an x86 machine, power supplies were engineered to provide significant amounts of
current on the +5V rail. Modern CPUs such as the Athlon and Pentium 4 run on the +12V rail. The problem is that many
power supplies are still based on older Pentium III-era designs and so even for many mid-range gaming systems,
chances are that the +12V component of the power supply is not going to be adequate.
This will all make sense when you actually calculate your power consumption budget. In the following table, you’ll find
estimates of power consumption for common devices that we obtained from AMD technical documents; they are a bit on
the high-end (but you’re better safe than sorry)
Current Consumption (Estimates)
+3.3V +5V +12V Device
Motherboard w/ onboard
3 2 0.3
High Performance fans
0 0 0.25 (including power supply
0 2 0 Memory (128MB DDR)
3 0 0 VGA
0.5 0.5 0 PCI Sound
0.4 0.4 0 PCI Network Card
High performance hard
0 0.8 2
0 1.2 0.8 CD-RW
0 1.2 1.1 DVD
0 0.8 0 Floppy
0 0.5 0 USB devices
0 0.25 0 Keyboard
0 0.25 0 Mouse
0 0.5 0 PCI Modem
0 1.6 0 FireWire
Athlon and Pentium 4 CPUs run entirely on the +12V rail and you can determine their power consumption using the
following formula that takes into account voltage regular inefficiencies:
1.25/12*core voltage* current
The processor’s current can be determined by reading AMD and Intel PDF tech documents, but as a general rule of thumb, the higher
the raw megahertz, the higher the current, and the smaller the die process, the lower the current. We’ve listed some common CPUs
+12V Current CPU
5.742188 Thunderbird 1GHz
6.289063 Thunderbird 1.1GHz
6.466667 Thoroughbred 2100+
6.55 Thoroughbred 2200+
6.872396 Palomino 1800+
7.115625 Thoroughbred 2400+
7.115625 Thoroughbred 2600+
7.291667 Palomino 2000+
7.492188 Palomino 2100+
8.145 Northwood 2GHz
8.505521 Northwood 2.4GHz
8.53526 Northwood 2.2GHz
9.292188 Northwood 2.6
9.707813 Northwood 3.06GHz
10.1175 Northwood 2.8GHz
It’s worth noting that although Pentium 4’s typically run cooler and have lower voltages than AMD CPUs, their higher raw
clockspeed means that they draw more power. We’re not sure why the 3GHz P4 draws less power than the 2.8GHz
model, but that’s what Intel’s documents list…
Power budget (cont’d)
With those guidelines in place, you can determine your specific power consumption. Let’s take a typical mid-range
gaming Palomino Athlon XP 2000+ system:
Athlon XP 2000+ system + Four fans
- power supply fan
- CPU fan
- Front cooling fan
- Rear exhaust fan
512MB DDR RAM
AGP Graphics card
PCI Sound Card
2 Network cards
2 IDE drives
Sum everything up and that’s
7.3 amps on the +3.3V rail
16.3 amps on the +5V rail
14.5 amps on the +12V rail
We know Power = Current * Voltage, and so by the math our minimum power spec is 279W. So all 300W power supplies
should be good enough right? And if we wanted to account for “overzealous advertising” we’d be “for sure” safe with a
350W power supply right? Not to mention, those numbers assume that every single drive in your system is being used at
the same time, and we’ve already said that the individual components err on the high-end side…
Well not exactly.
At FiringSquad, we don’t think you should expect to sacrifice stability when your system is under full load and so for the
rest of this article, we’ll continue to assume 100% power consumption on all devices. That said, because it is rare to have
every component active, a reasonable approximation is to assume 80% of your components are active at any given
instant and that your CPU is always drawing full power.
Manufacturers can bend the truth when it comes to power supply ratings and you never want to be so close to maximum
capacity all the time. At FiringSquad we like for power consumption to be at 80% of advertised maximum and so we need
to alter our numbers to account for that. Before I go on, let me say that this is personal preference. We are willing to pay
the extra cash for the extra piece of mind, but you don’t need to do this step. But for now, our power demands are:
9.125 amps on the +3.3V rail
20.375 amps on the +5V rail
18.125 amps on the +12V rail
349 W total
So a 350W would be just fine right? We’ve added a fudge factor for misadvertised products right? No, you need to look at
the individual rails, remember?
A Straightforward Example
Antec makes great power supplies, so let’s look at them. The ~$50 Antec 350W (SL350) power supply has
28A on +3.3V
35A on +5V
16A on +12V
Based upon these specifications, the Antec offers more than the minimum spec, and so in most cases the system will be
very good. It’s not the “80% of efficiency” that we’d prefer, but it should be all right.
The Antec SL350 power supply Note the power figures on the power supply
A Tricky One
What about a 265W Enermax EG265P-VE?
22A on +3.3V
25A on +5V
18A on +12V
This offers plenty of power on all three rails, and in fact it’s even better for the +12V rail, but remember, we need 279W at
peak use. Here, our peak is 265W. Not enough. The 300W Enermax has the following spec:
28A on +3.3V
30A on +5V
22A on +12V
Now, the individual rails are good enough for us and the total wattage is sufficient for our minimum. If you didn’t want the
extra piece of mind at running 80% maximum efficiency, the Enermax seems to be a good choice. If you wanted the
extra piece of mind, the 350W Enermax will give you top-notch performance.
Generic Power Supplies
That said, most of us don’t buy power supplies independently. We get case/powersupply combos – I mean, why spend
$50 on a power supply when you can get a case+power supply for $50? Let us take a look at the Foxconn Mid-Tower
with “AMD Listed 300W power supply” that a store like newegg has for $30.
20A on +3.3V
30A on +5V
10A on +12V
The 3.3V and 5V rails are great, but the 12V rail is too weak for our system! Under heavy processor load, Athlon XP
system would be very unstable and likely spontaneously reboot. What about another 300W power supply such as the
Codegen for $30?
20A on +3.3V
25A on +5V
10A on +12V
Still not enough. Let’s go up to 350W generic cases. $38 gets you a nice looking Foxconn setup with “350W AMD
recommended power supply up to XP2100+”
24A on +3.3V
35A on +5V
12A on +12V
Better, but the 12A still isn’t enough for our system since we have too many drives. What about a $56 400W generic
24A on +3.3V
35A on +5V
12A on +12V
What the heck? A 400W and 350W power supply offer the exact same specs? Well no, the individual “power pumps” for
the +3.3V and +5V and +12V are the same, but the pump at the power source is bigger.
What about a generic 450W power supply?
By now, you might think that “buying a generic case is just a bad idea.” In that case, lets look at Newegg’s $68 Chieftec
with a 450W “AMD approved” Austin power supply.
The +12V is just good enough to meet your bare minimum spec, but don’t think about adding extra hard drives. This
power supply is capable of providing a lot of juice to the +3.3V and +5V rails, but you really have nothing useful there. It’s
the 12V you’re worried about and this 450W only provides a maximum of 180W on +12V.
So what does this mean? When you shop for power supplies, you should always watch for the +12V lead. The majority of
low-cost power supplies you see at computer shows tend to have +12V rails with 10 or 12A. No matter how many total
watts the power supply may have, you’re setting yourself up for instability if you buy that power supply.
Your plan should be to first calculate what your power needs are. Use our data to help you, keeping in mind that in all
cases, we’ve erred on excess and that there are places to add “extra” headroom 3 different times.
1) The quoted power consumption for components is on the high-end
2) It’s rare to have every component drawing full power
3) We prefer not to run our power supplies at the limit. It reduces the stress on the component and leaves room to
Our approach just gives you the absolute piece of mind that your power supply is not going to be the source of any
system instability even when consider the power loss that occurs through conversion, and overzealous specifications.
Only you can decide which factors you want to take into account. Even then, it’s important to take this step since you’ll
find that there will be many power supplies with inadequate power even if you make no adjustments.
There are also some other lessons to be learned. We’ve all been taught that heat is the enemy of stability, and this is
true. It’s true not only for CPUs, but for hard drives and graphics cards as well. If you look back at the worksheet,
however, cooling fans run on the +12V rail. If you add too many fans, you could in fact be reducing the stability of your
overall system unless you have the power supply to back it up.
A CPU running at 46 C versus 40C is not going to be any less stable. However, if it took you plenty of exhaust fans and
blowports to get that performance, and you end up with too much demand on the +12V rail, you’ll have an unstable
system. This isn’t intuitive at first, but it’s something we’ve experimentally confirmed. You shouldn’t add extra fans if you
don’t have the power to support them.
Bottom line: Read the label the next time you pick out a power supply. Retailers that cater to enthusiasts such as
Newegg and Googlegear often take the time to provide images and/or the crucial power figures for each of their
products. That way you can shop and find the power supply that's appropriate for you.