Power is measured in Watts and P = V x I = E x f
Gate current I = Static Current (leakage) + Dynamic Current.
Early CMOS (VCC 5 volts): negligible static current,
but today at VCC of 1.3 volts it's 30 percent of consumption.
Dynamic current = Short circuit current + Dynamic charge current.
Dynamic charge current computation:
Activity ratio, a: percentage of clock cycles that see a transition.
The net toggle rate = Operating frequency of the chip (f) \times ~ a.
For leakage and short circuit power see cell library.
Further details: »Power Management in CPU Design.