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Chapter 18. Power Supplies > Power-Use Calculations

Power-Use Calculations

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 Unfortunately, these calculations can be difficult to make accurately because many manufacturers do not publish detailed power consumption data for their products.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. Table 18.26 shows the range of power usage for typical PC components I’ve observed over the past few years.

Table 18.26. Power Consumption Calculation
ComponentPower UsageComments
Motherboard50W–75WDepends on the number of integrated components.
Processor25W–150WFor each physical processor (not cores). Most are 50W–100W.
RAM5W–15WFor each module (DIMM).
Integrated video5W–15WIntegrated into the North Bridge chip.
Discrete video card25W–300WFor each video card.
PCI card5W–15WFor each nonvideo card.
PCIe card10W–25WFor each nonvideo card.
Hard disk drive15W–30WFor each drive. Power use increased during startup.
Optical drive15W–35WFor each drive.
Cooling fan3W–5WFor each fan.
USB/FireWire2W–5WFor each used port.


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