PSU: Power Supply University, Does Your PSU Make the Grade? - Finding Out What Wattage You Need
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If you wanted to determine the voltage of a device, lets look at some numbers here. I have a LG 4163B DVD burner that has the following information on its label: 5V / 12 V = 1.5A / 2.0A. What does this mean? It means that it will use 5 volts at 1.5 amps and 12 volts at 2.0 amps. What we need to find out is its needed wattage to function properly. To do this we multiply the separate voltages by its ampere and add them together. Multiplying 5 volts by 1.5 amps equals 7.5 and multiplying 12 volts by 2.0 amps equals 24. Adding these numbers together (7.5 and 24) gives us 31.5 watts of power consumption. This doesn’t mean that the optical drive will always use 31.5 watts of the power supplies output for its purposes, but it can when running at full speed. Ideally, then, you want to factor in a need of 31.5 watts for this DVD burner for those times when it will use that much.
For other devices that don’t have detailed information like that available, here is a generic “best guess” table to determine your system’s possible power needs:
Component Approximate Wattage |
Processor | 70 to 140 W + |
RAM per stick | 20 W per 256 mb |
Motherboard | 20 to 40 W |
Video card | 25 to 50 W + |
PCI card | 5 to 10 W |
Optical Drives | 10 to 20 W |
Cooling Fans (Case and CPU) | 2 to 5 W |
Hard Disk Drives | 10 to 15 + |
Keyboard | 1.5 W |
Mouse (Optical) | 1.5 W |
USB devices | 5 to 10 W |
Floppy drives | 5 W |
These numbers are rough approximations based on only a small selection of devices, and it is always best to find the exact information from the supplier of the component whenever possible.
When looking at a power supply and doing the mathematics necessary to determine absolute output, you will often find that its total wattage is much higher than advertised. This true, because there is no power supply unit that can provide maximum current on all its voltages at the same time.
Advertised wattages are often rounded to an even number and are not always accurate of the actual output. When looking to purchase a new power supply take into consideration that the actual wattage can be anywhere from 20 to 40 watts less than the packaging indicates. Bearing this in mind it’s never a bad idea to buy a higher wattage power supply than what you need to provide not only upgradeability for other devices but also allowing for discrepancies in reality and advertising.
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