http://www.tpub.com/doeelecscience/electricalscience2123.htm
A voltamp is a unit of apparent power, which is the combination of voltage and current in an electrical circuit. A watt, on the other hand, is a unit of real power, which is the actual power consumed by a device. The relationship between voltamps and watts is that in an ideal circuit with no reactive components, the apparent power (voltamps) is equal to the real power (watts). However, in real-world circuits with reactive components like inductors and capacitors, the apparent power can be greater than the real power due to the presence of reactive power.
A VAr meter only measures the reactive (imaginary) power. Apparent power is a combination of real and reactive power; thus having a VAr meter will not suffice to measure apparent power. Likewise, because the VAr meter only measures reactive power, it does not provide any information on real power.
The ratio of active power (real power) and apparent power is called power factor ( pf ). Power Factor ( pf ) = Active Power / Apparent Power = .................. ( kvar )
real power is to do do work and it is supplied to the load ,where as the reactive power is used for magnetising and the apparent power is the total power
Apparent power is VA. Real power is W reactive power is VAR. Under an inductive+resistive load the VA is higher than W
Use a wattmeter, as it only reads 'real power' of your load. Use an ammeter and a voltmeter, and the product of the two readings will give you 'apparent power' of your load. Since apparent power is the vector sum of real power and reactive power, use the following equation to find the reactive power of your load: (reactive power)2 = (apparent power)2 - (real power)2
Apparent power must be used for this is the total amount of power you need to generate. Apparent power includes all the loses of energy while real power is the power used by the electrical appliances. If you are talking about environmental impact then apparent power is the one you need because this is the overall power needed.
According to Wikipedia, all forms of power can be expressed in Watts, but typically one expresses only REAL Power in Watts (& Kilowatts).The standard is to express Apparent Power in Volt-Amperes (VA) (& KVA).The kilowatt is the unit of measure for true power; apparent power is measured in volt amperes.However, the volt ampere and the reactive volt ampere (for reactive power) are traditional units, used to help differentiate between apparent, true, and reactive power. SI recognises the watt as the unit for each.
Real Power: The actual power in Watts or K-Watts in AC or DC Circuits Apparent power: The Power in Inductive or Capacitive Circuits have Phase Lag & Lead measured in Volt Amperes VA or Kilo Volt Amperes KVA
power factor=real power/apparent power... real power in the sense the capacity of the circuit for performing a work in a particular time..for example the work done by a motor circuit is so and so...real power is what we consume from them... apparent power if the product of the current thro' the circuit and the voltage...it will be always greater than the real power.. if the POWER FACTOR IS 1.0(100%),THEN THE AC POWER IN THE CIRCUIT WILL BE EQUIVALENT TO THE POWER OFFERED BY A DC CIRCUIT....
The vector-relationship between apparent power, true power, and reactive power is represented by a right-angled triangle, whose hypotenuse represents apparent power and whose adjacent represents true power. Since power factor is defined as 'the ratio of true power to apparent power', you will find that this ratio corresponds to the cosine of the angle between them.
Energy meters monitor the supply voltage and the in-phase component of the load current -in other words, they read true (real) power not apparent power which, when time is taken into account (i.e. by the rotation of the meter's disc), results in a measure of energy consumed.