Estimating Computer Power Supply Wattage

But First, the Complicated Bit

I’m going to keep this as simple as I can, but there are one or two things you should know before we start.

A modern (less than 10 years old) computer’s power supply unit (PSU) is designed to supply five voltages at varying power capacities. The voltages are +3.3V, +5V, -5V, +12V and -12V. Because the computer has different power requirements at each voltage, the PSU design is optimised to provide the appropriate power to each rail. A rail is basically a single voltage, so there will be a +3.3V rail, a -5V rail, a +5V rail and so on. If you count the rails you’ll see it doesn’t match the number of wires exiting the PSU. Don’t worry about that – it’s just for technical convenience.

As technology and miniaturisation plodded on, so the operating voltage of the components reduced (smaller components require less voltage). As more and more components became lower voltage, the power requirement at that lower voltage increased. Without getting too technical, it became more efficient and practical for the motherboard manufacturers to convert the 12V rails to the actual voltages required at the component locations on motherboard. Transporting the power around the motherboard at this higher voltage (and, therefore, lower current as we’ll see in a moment) enabled the motherboard conductors to be narrower (roughly an eighth of the width) and, therefore, more densely packed. The reason for this is to be found in the power calculation:

P (power in Watts) = I (current in Amps) x V (Volts)

for the purposes of this explanation, I’ll turn that round to give us the current (I) as the result of the equation:

I = P / V

e.g. at 3.3V, a CPU requiring 65W will draw a current of 65/3.3 = 19.69 Amps. Thats almost 20 Amps going through those thin conductors!

At 12V, however, the current will be a much more manageable 65/12 = 5.41 Amps.

Okay, so how does this affect you? Well, older PSUs of the same wattage as newer ones may have a much lower power rating on the 12V rails. It would not be a good idea, therefore, to cannibalise an old PC for its otherwise perfectly serviceable PSU to use in a newer PC. The chances are, you would be overloading and overheating it to some extent. Conversely, a new PSU used to repair an older computer may not supply sufficient power at the lower voltages, again risking overloading. More important here is that the -5V rail is no longer used on motherboards, so new PSUs tend not to provide a -5V rail. If you’re using a PC that old, you’d be better off replacing the motherboard and processor, but that’s another story.

Now we’ve got that out of the way, let’s take a look at how to determine the wattage of the PSU we need.


If you have an Ecosaver Energy Meter (link below) then you can simply plug in your PC and read off the Watts. This is also a useful gadget for seeing where you’re wasting electricity around the house, seeing how much an appliance costs to run, etc. But perhaps your power supply unit (PSU) is dead or you just want to add it all up. Well, that’s just as easy, and explained on the next page.


The power usage figures are not always that easy to find on the respective manufacturers websites, but safe rules-of-thumb are what we need and to this end PC Power & Cooling have an excellent power reckoner (link below) on their website, although I think their ratings are a little high – they do, after all, have a vested interest in selling bigger PSUs – it doen’t hurt to overestimate. Allowance is built in to accommodate the fact that the various components, e.g. graphics cards may use more than one rail. Just work through the list and add up the numbers. Keep in mind that the balance of power requirements on the different rails may differ for older equipment. If in doubt, overestimate.

PSU rating

To determine the most efficient PSU for either method, divide the result by 7 and multiply by 10 (or just multiply by 1.43). The reason for this is that PSUs operate most efficiently when the load is at, or below, 70%. They will have a longer life and, more importantly, will save electricity (10% or more) over an undersized supply. Imagine how much it would cost to burn a 60W bulb all the time that your computer is switched on – this is how much you could potentially waste by not correctly sizing your PSU.

Here’s a worked example:

nVidia 8800GT PCI-E = 105W (source:nVidia)

Network card (average PCI) = 5W

DVD = 25W

Seagate Barracuda 500GB HD = 15W

Intel DP35DP motherboard = 105W (source: Intel)

Intel Core 2 Duo E8400 = 65W (source: Intel)

2 Gig RAM = 30W

Fans etc. = 6W

Total = 356W

70% calculation = 1.43 x 356 = 509W

So you can see that to run a computer constructed using these components you would be best fitting a PSU of at least 509W. A 550W or 600W would allow a margin for further upgrades.

Useful links

Ecosaver Energy Meter

PC Power & Cooling power reckoner


It’s not such a difficult task to estimate the requirements of your PC. It can be a little tedious if you need to track down specific figures, but in general if you are generous in your estimate and then, on top of everything else, you add a 10% or 20% margin for future upgrades, you can’t go far wrong. Whatever the rating of your PSU, the computer will only draw what it needs. If your PSU is an 910W monster and your PC only needs 350W, that’s all it will pull from the mains.

Some PC manufacturers, notably Dell, may not use standard connectors or may have an extra wire. If you are in doubt, check with your PSU supplier.

As mentioned earlier, you may have trouble sourcing a suitable PSU for an older motherboard. If this is your situation, I would strongly advise you to upgrade your motherboard – you can find part recommendations for several budgets here, and it can be fun and rewarding to do such a thorough upgrade.