You cannot simply calculate the power unless you know the power factor of the circuit. The simple equation works for DC or when the load is a perfect resistance (ie no inductance or capacitance). When inductance or capacitance is present (ie nearly always) the current phase either leads or lags the voltage phase such that the instantaneous power at any point on the voltage cycle is always less than it would be if the current was in phase (perfect resistance). The power factor is always <=1

Briefly put;

VA = V * A. This is the maximum theorectical power for a pure resistance.

Power = Power Factor * VA. This is the acutal disippated power.

A reasonable non-mathematical explanation can be found at http://www.microconsultants.com/tips/pwrfact/pfarticl.htm