Sorry, your design is not a watt meter (under AC), it should be a AC VA meter or a DC Watt Meter, it is because under AC supply the formula is V X A X Power Factor (PF for Water Heater = 1, PF for Motor = 0.5 ~0.7)
The measuring of power dissipated in a purely resistive circuit with a DC voltage source is trivial, as power equals voltage against current, P = VI. The math becomes a little trickier with AC-power sources, as the waveform dictates the averaging formula used to find the power dissipated. Circuits containing reactive parts, such as capacitors and inductors, on the other hand, are much more complex to measure.
Power Factor - Power Factor (PF) or Total Power Factor (TPF) equals the ration of real power (W), which performs the actual work of creating heat, light, motion, etc., over apparent power (VA) that is the combination of real power (W) and reactive power (VAR).