A little info taken from the
PSU FAQ (and written by me as well).
QUOTE
Q: How much power does a power supply use (both AC & DC)?
A: If a system consumes 200watts DC, then the power supply will only consume 200watts DC. It does not matter how many watts a PSU is rated for. -->For example if the system consumes 200watts DC but the PSU is rated at 500watts DC, the system will still consume 200watts DC(not 500watts DC).
Maximum DC power output can be affected by the PSU's internal temperature. Most manufacturers rate the maximum DC output at a internal temperature of 25C. This temperature is not a real-world scenario as most PSU internal temperatures are in the range of 35-40C+. As the temperature inside of a PSU increases, the maximum DC output decreases.
Because of the nature of switching PSU's, the amount of DC consumed does not equal the amount of AC consumed. Componnents inside a computer cannot utilize AC for power, so the AC needs to be converted to DC, thus "switching". In the switching process, some of the energy is lost being switched which also correlates to the efficiency of a PSU. Most PSU's have a efficiency rating of around the 70% mark on average. So, if a PSU (with 70% AC to DC efficiency) consumes 200watts DC, then it will consume 285 watts AC.
The easiest way to figure out how many watts a devices uses is with a watt-mater. After finding out how many watts a device uses, then consult your electric bill to see how much your power company is charging you on your kWh.
I can't say for sure how many watts your PC consumes because I am assuming you dont have a wattage meter like a Kill-A-Watt meter. Plus you haven't told us your system specs. Most 17" CRT monitors consue around 100watts AC when on. If you do leave your PC on 24/7, turn off your monitor when not using it.
For me, leaving my PC on 24/7 is costly since I live in California where electricity is expensive per kWh. If I left it on, I would say it adds about $10-20 per month to the electric bill on average.
