One thing to keep in mind with this configuration: you're not actually measuring true power. Rather by not using a voltage reference, you're calculating apparent power, which is measured in VA (Volt Amps) vice Watts.
For resistive loads, apparent power and real power are the same, or very nearly so. But with reactive loads, e.g. motors, computer power supplies, compact flourescent or LED lamps, etc. they will differ considerably.
