Usually the graphic cards will draw the most power when in use. Peek usage will cause them to really amp up their draw and when they are idle (as in just doing a basic desktop) they tend to draw much less power.
A large number of hard drives can cause a significant draw during boot up but not always during running. Some systems support a staggered spin-up for the drives to prevent the power supply from choking.
Video Card.
hard drive
the computer component not tested by post rutine is the power supply
Depending on your wants/needs, $750 - $2000.
Usually the power supply.
The OCZ StealthXStream is a great camping power supply.
because a computer Component can only Communicate in Power on (1)Power off (0) otherwise it is a quantum computer and there is no sutch thing.
the power supply provides electricity to each component to the computer and ensures that neither too much or too little power is delivered. it is generally box shaped with many wires coming from it which will be used to connect to each component of a computer
Good power supply and good cooling.
Uninterruptible supply
hard drive.
Each peripheral component draws a certain amount of power, so a PC technician may need to know how much power is needed for a component or many components on/in the computer. This would relate to making sure the Power supply has adequate output.