Have you actually measured the power consumption?
A 250 W PSU is rated to supply a maximum of 250W. It should have an efficiency of about 95% so the input will only be slightly higher.
In practice the consumption should be significantly lower most of the time. The full power will only be used when you are running the processor flat out, working the HD hard, burning a CD/DVD, and drawing loads of power from your USB ports.
Russell.