26.4 Choosing a Power Supply

Use the following guidelines to choose a power supply appropriate for your system:

Choose the correct form factor

Above all, make sure the power supply you buy fits your case and has the proper connectors for your motherboard. If your motherboard includes the ATX Optional Power Supply Connector, buy a power supply that provides that connector. Consider buying such a power supply even if your current motherboard does not require that connector, so that if you upgrade the connector will be available.

Match power supply to system configuration

Some sources recommend adding up maximum current draws for all system components and sizing the power supply on that basis. The problem with that method is that it can be nearly impossible to determine those draws for all components, especially motherboards and expansion cards. We recommend using the KISS method instead, as follows:

Basic system

For a desktop or mini-tower system with a Celeron or other low-end processor, 64 MB or less RAM, one IDE hard disk, one IDE DVD/CD-ROM drive, and zero or one expansion card, install a 230W to 250W power supply.

Mainstream system

For a desktop or mini/mid-tower system with a Celeron or FC-PGA Pentium III Coppermine processor, 128 MB RAM, one or two IDE hard disks, a DVD/CD-ROM drive, a CD-RW drive, perhaps a tape drive, and one or two expansion cards, install a 300W power supply.

High-performance system

For a mid- or full-tower system with one fast Pentium III, Pentium 4, or Athlon processor, 128 MB to 256 MB RAM, one or two fast SCSI hard disks, a DVD/CD-ROM drive, a CD-RW drive, a tape drive, and several expansion cards, install a 350W power supply.

Heavily loaded system

For a full-tower system with dual processors, 256 MB or more RAM, two or three fast SCSI hard disks, a CD-ROM drive, a DVD-ROM/RAM drive, a CD-RW drive, a tape drive, and all expansion slots filled, install a 400W power supply.

Obviously, individual configurations vary, but generally following these guidelines ensures that the power supply is adequate for the current configuration and has some room for growth if you add components. If in doubt, buy the next size up.

If you build a dual-CPU system, make sure the power supply you choose is rated for dual-CPU motherboards. Even power supplies that have relatively high overall wattage ratings are not necessarily adequate for dual-CPU motherboards. This is because many high-wattage power supplies provide much of that wattage at +12V DC, which is not usable by the processors. Two Intel Pentium III processors—and even more so two AMD Athlon processors—can easily draw 100W or more, which may exceed the maximum allowable combined current on the 3.3V and 5V rails. Our best advice for those building dual-CPU systems is to check the PC Power & Cooling web site (http://www.pcpowercooling.com) to locate a power supply appropriate for your configuration.

Match power supply capacity to case style

Regardless of your current configuration, take case style into account. It is senseless, for example, to install a 200W power supply in a full-tower case. You might just as well buy a smaller case, because that power supply will never support even a fraction of the number of devices the case can hold. Neither does it make sense to install a 450W power supply in a mini-tower case, which simply does not have room for enough drives to require all that power.

Avoid power supplies that are not AMD-recommended

AMD processors have historically had very high current draws compared to similar Intel processors. People who upgraded Intel systems by installing a new motherboard with an AMD processor often found that the new system would not boot or crashed sporadically because the existing power supply was inadequate. That problem was so common that AMD took the extraordinary step of certifying power supplies by brand and model for use with their processors. The unintended result of this was that AMD certification became a pretty good measure of the overall quality of a power supply. So, even if you're building or upgrading an Intel system and have no plans to use an AMD processor, it's a good idea to make sure that the power supply you buy is on the AMD-recommended list. Note, however, that many good power supplies do not appear on the AMD-recommended list. This doesn't necessarily mean that they're not sufficient to power an AMD processor, but may simply mean that AMD has not gotten around to testing them yet. (http://www1.amd.com/athlon/power)

Avoid ATX power supplies that are not Pentium 4 compatible

Any ATX power supply you buy should be ATX12V compliant. Even if you're not using a Pentium 4 in that system, buying an ATX12V power supply protects your upgrade path if you decide later to install a newer motherboard. We expect that an increasing number of newly introduced motherboards, both for Pentium 4 and other processors, will begin including the 4-pin +12VDC connector. If your power supply doesn't have this connector, you may need to replace the power supply when you upgrade the motherboard. (You can use an ATX12V adapter cable to pull +12VDC from a drive connector to the motherboard, but that doesn't guarantee that your original power supply will work with motherboards that require +5V_SB current higher than required by ATX 2.03.) Buying an ATX12V unit now avoids that needless expense later.

Avoid cheap power supplies

Don't assume that the power supply bundled with an inexpensive case or a $20 unit you find on the sale table at the computer store is adequate. It probably isn't. A good power supply costs at least $35 for a basic system, $50 to $75 for a mainstream mini/mid-tower system, and $100 or more for heavily loaded, full-tower systems.

Avoid replacing proprietary power supplies

Some big-name computer makers (notably Compaq and Dell) have used proprietary power supplies in their systems, although this practice is fortunately less common nowadays. If a proprietary power supply fails, the only source for a replacement is the manufacturer, who may charge literally $500 for a power supply equivalent to a standard $50 unit. If you need to replace a proprietary power supply in a system that cannot physically accept a standard power supply, it may be cheaper to replace the PC than to buy a new power supply for it. Rather than pay proprietary prices, consider buying a new case and power supply and migrating the components from the failed unit to the new case. You may also have to buy a new motherboard, but you should be able to migrate the processor, memory, and other components.

A similar problem exists with some of the inexpensive systems sold by such retailers as Circuit City and Best Buy. Many of those use inexpensive, low-output SFX power supplies that are prone to fail quickly. For some of these systems, manufacturer policies make replacing a failed power supply quite expensive, even if the system is still under warranty.

Instead of overpaying for a replacement power supply that's no better than the original, purchase a replacement power supply from PC Power & Cooling (http://www.pcpowercooling.com), which makes both a standard SFX power supply and a special SFX model for eMachines and HP 67XX models. Better still, replace the power supply before it fails. Doing so improves the system's reliability, stability, and life expectancy. Put the original, still-functioning power supply on the shelf as a spare, although you probably won't need it.