i was wondering.....why do alot of these computer towers have power supplys of 300 watts or so - do the hard drives and cpu, ect. really require that much power? it just seems like a lot of energy to run the electronics in these systems. i'm sure a laptop doesn't require anywhere near that much power.
in normal usage, does this mean that my computer would be drawing 300 watts of power or is it dependent on the processes going on in the system? is a higher rated power supply wasteful to the ordinary user? thanks.

no, you're not drawing 300 at all times. you want a supply to be ultra stable, the 5 volt and 12 volt lines to stay at a constant level no matter what the load is, the extra wattage capacity of the supply ensures that the levels don't dip when a sudden load is put on them. in a normal desktop you could probably get by with a 200 or 250, but they are basically the same price and size so there is little reason to buy the lower wattage one.

The total power is the sum of each component in your system. Therefore, the more stuff you have in the PC the more power you will need. A normal PC will not need 300 watts of power. It isn't wastefull cause the PC only uses the power it needs.

My AMD PC and monitor uses about 256 watts. That is based on the load from my APC 650 UPS (55.9%). UPS's are rated in VA not watts so to convert multiply by .707.

If I estimate that the monitor uses about 1 amp or 84 watts that leaves about 172 watts for the PC. All I have is a 120 gb hard drvie, a DVD/CDROM, a floppy disk drive and a case cooling fan. I didn't include power supply losses. You can look at the side of your power supply to see how much current each ouput (5 VDC, 3.3 VDC, +12VDC and -12 VDC) can source. My DVD/CDROM uses 1.2 amps for +12 VDC and 1.2 amps for +5VDC. So that is 21.9 watts.

Watts = Volts * amps.


A laptop uses low power components plus apm which shuts components off after a set time period or inactivity.


I have simplified things a little but you can find out the current required for each component and calculate the watts and add them up to find the total. The power specifications are not listed for my Soyo motherboard so no clue on what it requires.

thanks for the info. i want my next computer to be scaled down as i don't need all the expansion bays ect. my old box looks nice and is smaller so i'm thinking of making a project out of it. thanks again.

This is quite a complicated question and having done a lot of overclocking on AMD systems, power supply output and quality of output is very important on the AMD K7 and Intel P4. Firstly the rating of the power supply can be ambiguous. Both cpu's can be very hungry for power when the system is under full load e.g. when loading the OS. The AMD K7's are spec'ed to draw 40+ amps under full load. Now comes the complicated part. As explained above the power supply has 2 circuits a 12V and a 5V. The processor derives its source from the 5V. Looking at the max power required under full load = 5 x 40 = 200W. There are no 300W PSU's that I know of that supply 200W on the 5V line. Poor quality power supplies can be blamed for many "dead motherboard" failures. i.e. the system fails to POST and the motherboard is blamed. As a general rule if one intends using a AMD K7 or P4 then the recommended minimum output would be 200W on the 5V line. This is usually only available on 400W+ PSU's. The specifications of most PSU's are available on the side of the PSU.
CPU's used in mobile systems have different power requirements to desktops and are specially manufactured for this purpose by both AMD and Intel.