Hardware Tips: Do the Math to Get Your PC All the Power It Needs

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Your PC may be dying of starvation. CPUs, graphics cards, and other components have a voracious appetite for power; and an overtaxed power supply can cause performance hiccups, sudden freeze-ups due to memory errors, and even premature death from overheating.

When you add memory, a second hard drive, or a high-end graphics board to your system, you risk pushing it over the edge. Even a midrange PC can be stressed by a demanding upgrade: The GeForce 6800 Ultra graphics card from NVidia can draw over 100 watts at peak, leaving a system equipped with a power supply of 300 watts or less severely undernourished.

Underpowered Upgrades

To calculate how much power your PC needs, total the wattage requirements for each of its components, and then add a safety margin of 30 percent. Our chart shows the typical energy use of common components. For a more detailed summation of your system's wattage needs, use the numbers in the Power Wattage Calculator at the JS Custom PCs Web site.

Compare your total to your power supply's maximum wattage rating, which should be printed on the outside of its case. The rating can also be found in your PC's manual or on the vendor's Web site.

If the number of watts your PC needs (plus the 30 percent safety buffer) is less than the maximum wattage rating of your power supply, your system may be adequately powered. However, wattage numbers don't tell the whole story. Different components require power to be delivered at different voltages: 3.3V, 5V, or 12V. If electricity were water, the voltage streams would correspond to an eyedropper, a household water tap, and a high-pressure fire hose. Today's power-hungry components need more 12V power than older hardware does, so a power supply must provide adequate 12V current. A supply's capacity to provide 12V power is measured in amperes, or amps. Its specs should tell you the amps the product can deliver at 12V, 5V, and 3.3V.

Adding up the 12V power requirements of each component is difficult, however. Vendors rarely publish this data, and some components use power at more than one voltage. For example, a hard drive may use 12V power to spin its disks and 5V power to operate the disks' circuitry. Consult the power supply article at FiringSquad for more information on calculating the voltage requirements of your equipment.

Power Lifts

To replace a power supply, first unplug the PC's power cord, ground yourself with a wrist strap or by touching a water pipe or other grounded object, and open the case. Disconnect the power supply's lines from the motherboard and other components, remove the screws holding it in place, and lift it out. Then reverse the process to install the new unit.

Don't pinch pennies: PC components need consistent power that's free of distortion and noise. Power supplies priced at less than $40 are likely to be more prone to voltage fluctuations than models costing $60 or $70. Paying more for a unit from a well-known maker such as PC Power and Cooling or Antec can save you both time and money in the long run.

Get a good fit: Most desktops made in the last six years fit either the ATX case style or the smaller SFX style. If you're unsure which type of case you have, check your PC's manual, or remove the old power supply and make sure the new power supply's measurements are the same.

Count your connections: The new power supply must have the same connectors as the old one, plus any connectors needed for future upgrades. Note that ATX motherboard connectors can have either 20 pins or 24 pins. Again, check your PC manual or the motherboard maker's Web site for this information.

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