Power and Energy Measurement

A wattmeter has a current coil in series and a pressure (voltage) coil across the load, so it reads $VI\cos\phi$. Three-phase power is measured with three wattmeters (four-wire) or two wattmeters (three-wire).

Energy meters: older electromechanical induction meters used a rotating aluminium disc; modern static meters sample voltage and current digitally, compute $\int vi\,dt$ and record kWh, kVArh, maximum demand and time-of-day data. India is rolling out prepaid smart meters with remote communication.

Meter constant example: "3200 imp/kWh" — 3200 LED flashes per kWh.