power is the product of voltage and current.so we can
measure the value of power just by using ammeter and
voltmeter.......then why do we use a wattmeter?

Answer Posted / nageshwaran.n

No doubt, we could use Voltmeter & Ammeter for calculating
Power but only Apparent Power that too at that instant.
Apparent power = voltage X current,

Actual RMS Power (real power) = voltage X current X power
factor

And to measure real power we need Wattmeter only.

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