You will need hardware to accurately monitor in real-time.
But if you're just looking for an average usage figure, you could just add up the power usage figures provided by the manufacturers.
For a very rough guide, my PC for example uses about 120 (normal) -180 (peak) watts for the base unit, and another 60 or so for the monitor, and that is for a
Q6600
4gb of ram
PkP Premium
4 SATA drives
3870
2 optical drives
20" LCD