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My desktop didn't turn on so I took it to Geek Squad.

It turned out the power supply was not working. The technician provided a new 400 watt PSU to replace the broken 300 watt one.

Is it a problem if I use 400 watts? What's the advantage/disadvantage of using more watts (apart from electricity bill)?

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The main advantage of a 400 watt power supply is that you would be able to have more devices (such as Blu-Ray, DVD-ROM, hard disks etc) within your desktop if required.

The only disadvantage would be the potential to use more power but that is often offset by more efficient power supplies, 'green' technology (e.g powering down drives etc that aren't in use.)

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Power in watts = Voltage in volts * Current in amps

As long as the output voltages of the two power supplies are the same, there is no disadvantage in using one with a higher wattage. Using the equation above, the higher wattage will allow the power supply to provide more current, so you can add more devices to your computer.

Assuming the two power supplies have the same efficiency, there is no disadvantage or additional energy used with the bigger power supply if they are powering the same system. (Since the "400 watt" rating refers to the maximum power, the actual power would be the same)

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"no disadvantage ... with the bigger power supply" -- Not quite true. PSU efficiency is not a constant for all levels of power output. Efficiency starts to go way down below about 20% of max power; 80Plus certification does not bother to measure below 20% of max power. So actual consumption from the wall outlet will vary for different PSUs of the "same efficiency" for low wattage during sleep and idle modes. And W = vi is only true for DC. For AC there's a power factor coefficient. –  sawdust Jan 16 '13 at 7:30
    
@sawdust Ah, I see. I only design DC/DC power supplies. :-) –  Kevin Chen Jan 16 '13 at 7:33
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