Todays systems and power supply units only get what they need, nothing more. If you have a 500 Watt PSU and your entire system needs 250 Watt in total, it will only draw just that.
If you use old(er) computer components they might always ask full power on anything.
Thanks to energy saving components (if your cpu is doing nothing, it won't draw much power, harddisk sleep mode, etc) the power consumption these days is greatly recuded.
What kills most systems isn't really the power consumption, but the unstable currents that cheap PSU's give off.
12 Volts needs to be 12 Volts precisely (or put it scientific: 12.000). If the currents fluctuate too much and/or are below the specifications you can get all kinds of strange errors on your system.