You will need
  • tester;
  • - clamp;
  • - a Handbook on electrical engineering;
  • calculator.
Measure with a tester the voltage of the network to which the connected appliance.
Measure using a clamp meter to the current value.
Voltage - permanent
Multiply the amount of current (amperes) the value of voltage (volts). The resulting work is the power in watts. For conversion to kilowatts you need to divide that number by 1000.
Voltage — single phase AC
Multiply the voltage by the amount of current and the cosine of the angle Phi (coefficient of power). The resulting work - consumed active power in watts. To translate that number into kilowatts divide it by a thousand.
The cosine of the angle between the full and active power in the triangle of capacity equal to the ratio of active power to the full. The angle Phi is sometimes called the phase shift between voltage and current - shift occurs in the presence of circuit inductance. Cos Phi equal to one if purely resistive loads (electric heaters, incandescent lamps), and about 0,85 — with a mixed load. The less reactive component of full power, the lower the loss, so the power factor are striving to improve.
Voltage — three-phase alternating
Multiply the voltage and current of one of the phases. Multiply the resulting value by the power factor. Similarly, calculated power of the other two phases. Then, all three phase power fold. The resulting amount will be the value of electrical power connected to three-phase network. With balanced load on all three phases active power is equal to the product, phase current, phase voltage and power factor multiplied by three.