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Power Supply Reference: Consumption, Savings, And More

Tom's Hardware and Que Publishing are partnering up to give you four chapters from Scott Mueller's Upgrading And Repairing PCs, 20th Edition. This sixth and final installment is the second half of the forth chapter we're making available from Scott's book, which covers Power Supply Usage Factors. Don't forget to check out the previous chapters published on Tom's Hardware, Computer History 101: The Development Of The PC, Hard Drives 101: Magnetic Storage, LAN 101: Networking Basics,  LAN 102: Network Hardware And Assembly and Power Supply Reference: Specifications.

When expanding or upgrading your PC, ensure that your power supply is capable of providing sufficient current to power all the system’s internal devices. One way to see whether your system is capable of expansion is to calculate the levels of power consumption by the various system components in your system, and then compare that to the rating on the power supply to see if it is up to the job. This calculation can also help you decide whether you must upgrade the power supply to a more capable unit. Unfortunately, these calculations can be difficult to make accurately because many manufacturers do not publish detailed power consumption data for their products. In some cases, you can find the specs from a similar component and go by that data instead. Usually components of the same basic design, capability, and vintage have relatively the same power consumption characteristics. The following table shows the range of power usage for typical PC components I’ve observed over the past few years.

Component 
Power UsageCommentsMotherboardDepends on the number of integrated components.ProcessorFor each physical processor (not cores). Most are 50 W–100 W.Integrated videoIntegrated into the North Bridge chip (Ed.: Though, increasingly on the CPU).Hard disk driveFor each drive. Power use increased during startup.

Of course, power consumption can vary greatly for different devices such as processors and video cards, so if you want to be more informed, consult the data sheets or technical manuals for your specific components. Also, these overall wattage figures do not give the breakdown covering which of the rails (+3.3 V, +5 V, or +12 V) each device will use. In some cases, the combination of components used can exceed the available power on a single rail while still being under budget for the total wattage available from all the rails combined. That is in fact one reason that people end up purchasing a power supply with a much higher watt rating than might seem necessary.

After you’ve added up everything I recommend, multiply the total power consumed by all your components by 1.5 to estimate the size of power supply required. This allows some headroom for future expansion and accounts for the fact that at certain times some devices can draw much more than their nominal power.

If you want an easier way to calculate your estimated power requirements, Asus has a fairly good power supply wattage calculator that you can use online at the following URL: http://support.asus.com/PowerSupplyCalculator/PSCalculator.aspx. After you fill in all the fields with the components in the intended system, the calculator gives you an estimate of the minimum power supply rating you should choose to power the system.

Different types of bus slots can provide different levels of power for cards. Fortunately, it is rare for any cards other than video cards to use the maximum allowable power. The table below shows the maximum power available per slot for different bus types.

Maximum Available Power per Bus SlotBus Type+3.3 V Current (Amps)+5 V Current (Amps)+12 V Current (Amps)Total Power (Watts)

The biggest cause of power supply overload problems has historically been filling up the expansion slots (especially with multiple video cards), using high-powered processors, and adding more drives. Multiple hard drives, optical drives, and floppy drives can create quite a drain on the system power supply. Be sure you have enough +12 V power to run all the drives you plan to install. Tower systems can be especially problematic because they have so many drive bays. Just because the case has room for the devices doesn’t mean the power supply can support them. Be sure you have enough power to run all your expansion cards, especially video cards. However, remember that most cards draw less than the maximum allowed. Today’s newest processors can have high current requirements for the +5 V or +3.3 V supplies. When you’re selecting a power supply for your system, it pays to be conservative, so be sure to take into account future upgrades or additions to the system.

Many people wait until an existing component fails to replace it with an upgraded version. If you are on a tight budget, this “if it ain’t broke, don’t fix it” attitude might be necessary. Power supplies, however, often do not fail completely all at once; they can fail in an intermittent fashion or allow fluctuating power levels to reach the system, which results in unstable operation. You might be blaming system lockups on software bugs when the culprit is an overloaded power supply. In addition, an inadequate or failing supply causing lockups can result in file system corruption, which causes even further system instabilities (which could remain even after you replace the power supply). If you use bus-powered USB devices, a failing power supply can also cause these devices to fail or malfunction. If you have been running your original power supply for a long time and have upgraded your system in other ways, you should expect some problems, and you might want to consider reloading the OS and applications from scratch.

Although there is certainly an appropriate place for the exacting power-consumption calculations you’ve read about in this section, a great many experienced PC users prefer the “don’t worry about it” power calculation method. This technique consists of buying or building a system with a good-quality 500-watt or higher power supply (or upgrading to such a supply in an existing system) and then upgrading the system freely, without concern for power consumption.

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Power Supply 101: A Reference Of Specifications

Tom's Hardware and Que Publishing are partnering up to give you four chapters from Scott Mueller's Upgrading And Repairing PCs, 20th Edition. This fifth installment is the beginning of the forth chapter we're making available from Scott's book, which covers Power Supply Fundamentals. Don't forget to check out the previous chapters published on Tom's Hardware, Computer History 101: The Development Of The PC, Hard Drives 101: Magnetic Storage, LAN 101: Networking Basics, and LAN 102: Network Hardware And Assembly. In the days to come, we'll also present a comprehensive look at Power Supply usage factors.

The power supply is not only one of the most important parts in a PC, it is unfortunately one of the most overlooked. Although most enthusiasts who build their own systems understand its importance, the mainstream PC buyer generally does not. Some that do pay any mind seem concerned only with how many watts of power it is rated to put out (even though no practical way exists to verify those ratings), without regard to whether the power being produced is clean and stable or whether it is full of noise, spikes, and surges.

I have always placed great emphasis on selecting a power supply for my systems. I consider the power supply the foundation of the system and am willing to spend a little extra to get a more robust and reliable unit. The power supply is critical because it supplies electrical power to every other component in the system. In my experience, the power supply is also one of the most failure-prone components in any computer system. Over the years I have replaced more power supplies in PCs than any other part. A malfunctioning power supply not only can cause other components in the system to malfunction, but it also can damage the other components in your computer by delivering improper or erratic voltages. Because of its importance to proper and reliable system operation, you should understand both the function and limitations of a power supply, as well as its potential problems and their solutions.

The basic function of the power supply is to convert the electrical power available at the wall socket to that which the computer circuitry can use. The power supply in a conventional desktop system is designed to convert either 120 V (nominal) 60 Hz AC (alternating current) or 240 V (nominal) 50 Hz AC power into +3.3 V, +5 V, and +12 V DC (direct current) power. Some power supplies require you to switch between the two input ranges, whereas others auto-switch.

Technically, the power supply in most PCs is described as a constant voltage switching power supply unit (PSU), which is defined as follows:

Constant voltage means the power supply puts out the same voltage to the computer’s internal components, no matter the voltage of AC current running it or the capacity (wattage) of the power supply.Switching refers to the design and power regulation technique that most suppliers use. Compared to other types of power supplies, this design provides an efficient and inexpensive power source and generates a minimum amount of heat. It also maintains a small size and
low price.

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Picking The Right Power Supply: What You Should Know

How does a power supply work? Why is it important to choose a sufficiently powerful and efficient model? We guide you through discussions of efficiency and tips for getting the best deal before we go on to explain why less can be more in the PSU market.

This piece is for the folks who'd like to learn more about about the facts, technologies, and terminology behind PC power supplies.

Don't worry; this won’t be complicated or boring. We’ll just quickly explain how a switching PSU works, then use examples to illustrate some of the most common technical issues. We’ll explain what efficiency, loss, and reactive power mean, and why those words are relevant to you. Then we’ll look at the possible and (more importantly) necessary protective measures before applying theoretical knowledge to practical examples.

Big versus small, efficient versus high-performance; we're going to examine three different PCs based on a trio of different usage models, calculate the power supplies they really need, and then explain the right class of PSU to use in them based on quality and long-term environmental impact.

Remember those ancient radios with the vacuum tubes? They were massively built and tended to be clunky, heavy, very functional-looking things. However, it wasn’t just the wooden frame contributing to their weight. The large, massive transformers added their part as well.

The point is, even back then, clever engineers were taking advantage of a neat trick of physics that would later come to be used in every modern switching power supply. In order to convert a high alternating current into a low one and achieve galvanic separation of currents, they used normal, albeit powerful, transformers with a core made of iron plates.

While a mains frequency of 60 Hz required a comparatively large transformer, the so-called output transformers that delivered much higher low-frequency signals between 100 Hz and 16 kHz could be built much smaller while handling the same power. By aggressively capping the frequencies at the lower end of the spectrum, it was possible to increase the power that could be handled by a transformer of the same size. With the invention and subsequent introduction of new components, such as powerful switching tubes, and later, semiconductors using the same underlying physical principle, this advantage was carried over into other fields, opening up new possibilities.

Due to the high overall power requirements of modern computers, a conventional transformer-based PSU is no longer capable of converting the mains power into the low voltages required by PC components. The transformer required for the job would be too large and consequently far too heavy. Instead, we use switching power supplies that employ the same frequency trick as the good old tube radio. Their job is to provide the required voltages and currents as efficiently as possible, while also reliably maintaining those levels. Analog (linear) solutions are no longer viable solutions. Instead, we now rely on transistors as switches to convert the mains power into higher frequencies, allowing us to use smaller transformers to transmit high power levels. Indeed, this is where the term “switching power supply” comes from. We’ll cover how those work in more detail in the next chapter. Don’t worry, it’s all simpler than you might think.

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Sub-$75 Mainstream Power Supply Roundup

In our last PSU article, we reviewed a bunch of relatively expensive high-performance devices suitable for powerful gaming computers and workstations. The average user typically prefers something a little bit more affordable, though, which is why today we're reviewing a handful of PSUs that can be found for around $75.

The potential reasons for buying a new PSU are many. Perhaps you've added a new graphics card, upgraded to a high-performance CPU, or are just replacing a faulty unit. A normal system usually doesn't require a high-end PSU. In most cases, the consumer is simply on the lookout for a cheap (and at the same time reliable) power supply at a reasonable price, so that's precisely what we decided to look at here.

The request we sent to manufacturers was simple: send us PSUs with a maximum street price of $75, regardless of wattage, features, or 80 PLUS certifications. What do you get in this price range?

Unexpected Early Goodbye

Our first test was originally made up of nine PSUs ranging from 400 to 500 W. We say "originally," because the AXP 500P12P died just as testing was getting started. With a load of 380 W, the 500 W PSU gave out a loud bang, sent sparks flying, and finally vanished in a smoky death. Since we were not expecting something like that at such an early phase in the testing, we'll give AXP the chance in a later article to prove that this was an isolated incident.

Then There Were Eight

Eight test candidates ultimately made it through the tests performed in our laboratory. Antec, Chieftec, Corsair, Enermax, FSP, Huntkey, Xigmatek, and a new AXP 500P12P were all included at capacity points between 380 to 500 W. That's enough power for most office and home entertainment computers. Despite a low street price of roughly $75, six of the PSUs even managed to achieve coveted 80 PLUS certifications. Antec, Chieftec, Corsair, Enermax, Xigmatek, and Huntkey all step things up with a Bronze-level cert. In order to earn that badge, PSUs must reach the following efficiency goals at 115 V: 82% at 20% of maximum power, 85% at 50% of maximum power, and 82% at 100% of the maximum power.

The AXP PSU can only do that under 230 V and thus cannot get certification. Similarly, the FSP PSU is only intended for 230 V applications and thus cannot receive the certification either. The pair of PSUs is completely useless here in the US. Nevertheless, our measurements will show whether they can still impress the European crowd.

Noticeable Differences In Quality

While we wanted to stay impartial before the testing, we couldn't help but notice the differences in quality during the unboxing process. Manufacturers tried to keep costs under control by skimping on the number of connectors, as well including shorter, lower-quality cables. None of these models feature a modular cabling design. Even the packaging is rather stripped-down. Further, the low weight of some PSUs at least suggests the use of cheaper components. This may not reflect immediately in testing. However, the power supplies might not last for as many years or support the same load strain as PSUs with higher caliber components.

Unfortunately, we did not get to see whether the PSU from AXP could hold its own against the other supplies, despite its lack of 80 PLUS approval. As with our first sample from the company, the second never even made it past the warm-up phase of testing. At a load of around 420 W (mind you, this is a 500 W PSU), several of the voltages rapidly dropped, the PSU produced some banging noises, sparks flew, and eventually the unit went up in smoke. After that, it wouldn't operate anymore and smelled like, well, fire. This video documents the failure of the PSU quite impressively. Check it out if you dig things going "pop."

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Soon At Tom's Hardware: Full-Scale Power Supply Testing

Often overlooked, the power supply unit (PSU) plays a crucial role in supplying the PC with stable and sufficient power. Whether you only need a 300W PSU for a modest small form factor box or a 1,200W model for a gaming juggernaut, the quality of your power supply will not only affect the system's performance, but also its longevity.

Bad power can mean early failures...and there are plenty of bad PSUs out there waiting for the unwary. This is why we felt it necessary to start testing PSUs in order for our readers to make better component decisions.

Test equipment plays an important role when evaluating power supplies though, which is why it's such a difficult job to do properly. Only with the help of highly precise measurement instruments can we document the differences between comparable models in an objective and reproducible way. Tom's Hardware performs all of these tests in professional labs, just like the ones found in PSU manufacturing facilities, where we get access to equipment from Chroma. For those new to this space, Chroma is the de facto vendor of PSU testing equipment.

When testing, we measure the following details:

Standby power consumption at 230VInrush current at 115V and 230VEnergy efficiency at 20, 50, and 100% of the stated maximum sustained load at 115V and 230V (according to the 80 PLUS consortium's certification)Energy efficiency at 25W, 50W, 85W, 300W, and 500W of power draw at 115V and 230VHold Up Time after losing power input at 115V and 230V12V rail overload test at 110% of the specified maximum sustained loadShort circuit test at 115V and 230VMeasurement of the temperature difference between intake and exhaust air (maximum delta) at 115V and 230V

In addition to the above mentioned values, Tom's Hardware also takes into account the ergonomics of the PSU and its accessories. This includes features like the the length and design of the cables, cable modularity, and any useful accessories. All important technical data will be presented in a table.

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