PSUs fluctuate depending on the needs of the system. Your PSU should always have some headroom to allow for sudden changes of output needed by the psu by the rest of the system, as in hardcore gaming for example.
But as Shum said, you wont be at full-bore all the time.
*Interesting sidenote: Even with the PC off, there will always be a voltage present to the motherboard, (+5 volts), which is used to keep the system at the ready so when you push in the front case switch, the system will start up. A signal is sent to the CPU which tells the PSU to start everything up. But that small constant voltage in itself wont run up your light bill. Your coffee pot or waffle maker uses tons more electricity than your PC.