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 Unfortunately, these calculations can be difficult to make accurately because many manufacturers do not publish detailed power consumption data for their products.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.
ComponentPower 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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