First – a simple question
Right or Wrong? Power = Voltage x CurrentThat statement is correct for DC systems but there are two major complications for AC systems.

RMS or Effective Value
Peak values in the alternating voltage or current curves only lasts a short instant. They are not really representative of the ability of the voltage and current to do work and thus they are not used in Power Calculations.Scientists use a statistical method to define the effective values. It is called the RMS or RootMeanSquare values. The result of the definition is that:Veff / rms = 0.707 x VpeakThe same applies to current too.TIP: You can reasonably assume that all the Voltages and Currents reported by a Power Meter are reported as RMS or Effective values unless otherwise indicated.
TIP: Most multi meters report RMS values

Apparent Power
Apparent Power is the power delivered by a power source to a load like a motor. In almost all real world situations that use AC, you need to supply more power (Apparent Power) to a device than it will do work (Real Power).The (vector) difference between the two represents the work done to overcome the inductive and capacitive effects of the load.Apparent Power is measured in units of VA – Voltamperes. These are actually Watts but we use the new unit name to reduce confusion. Thus when you see VA on a data sheet you can reasonably conclude that they are talking about Apparent Power.Apparent Power is calculated:S(Common symbol for Apparent Power) = Veff / rms x Ieff / rms – Single Phase Calc

Power Factor and Phase
Power Factor is calculated: PF = Cosine ( phase angle in radians)Power factor has no engineering units.The value of PF ranges from 1 to 0 to 1 (lagging – none – leading)Loads that only present a resistive load (no capacitance or inductance) have a PF of 1.Inductive Loads
Capacitive Loads

Real Power and Reactive Power
Think of Real Power as useful power – a measure of how much work is being done.The units of Real Power are Watts.Real Power is calculated : P(Real) = S(Apparent Power) x pfReactive Power is the (vector) difference between Apparent Power and Real Power. The energy used to produce the Reactive Power is stored in the magnetic/electrical field of the Inductive Load. In the case of the capacitive load the magnetic/electrical field of the Inductive Load produces the Reactive Power. Reactive Power cannot be harnessed to do useful work.Reactive Power is identified by the symbol : Q
The engineering units of Reactive Power are VAR – Voltamperes Reactive. These are also Watts but we use VAR so that we know we are talking about Reactive Power.

Diagram: RMS measurement
Diagram: Leading / Lagging
THD – Total harmonic Distortion (also called Distortion)
In simple terms THD is a measure of distortion reported as %. If a device (any active device but think of rectifiers, variable speed drives … as practical examples) is given a sine wave as in input the output is never a faithful 100% reproduction of the input. A series of harmonics of the original wave distort the original wave form. The THD % is an attempt to ‘numberize’ the degree of distortion to allow for comparison. The % number is somewhat controversial because some harmonics are more important than others and there is no weighting.THD(%) = 100 * SQRT[(V22 + V32 + V42 + … + Vn2)] / VtWhere V2, V3 are the RMS values of each voltage harmonic and Vt is the total RMS output voltage.

Sag / Swell or Dip / Surge
Duration is 0.5 cycle and greater. Voltage sags are the most common power disturbance. Voltage sags can arrive from the utility. In most cases, sags are generated inside a building. For example, in residential wiring, the most common cause of voltage sags is the starting current drawn by refrigerator and air conditioning motors.Sags do not generally disturb incandescent or fluorescent lighting. motors, or heaters. However, some electronic equipment lacks sufficient internal energy storage and, therefore, cannot ride through sags in the supply voltage. Equipment may be able to ride through very brief, deep sags, or it may be able to ride through longer but shallower sags. 
Under / Over Voltage
Over Voltage is an increase in effective voltage to more than 110% for longer than one minute. Under Voltage is a decrease in effective voltage to less than 90% for longer than one minute. Take care with this definition because it tends to change from vendor to vendor. 
Transient Voltages / Spikes / Surges
Refers to short duration (less than 1 cycle) events. Low frequency transients are often called “capacitor switching transients”. High frequency transients are often called impulses, spikes, or surges. They can be caused when a discharged powerfactorcorrection capacitor is switched on across the line.High frequency transients are caused by lightning, and by inductive loads turning off. Typical rise times are on the order of a microsecond; typical decay times are on the order of a tens to hundreds of microseconds. Often, the decay will be an exponential damped ringing waveform, with a frequency of approximately 100 kHz.Extremely fast transients, or EFT’s, have rise and fall times in the nanosecond region. They are caused by arcing faults, such as bad brushes in motors, and are rapidly damped out by even a few meters of distribution wiring. Standard line filters, included on almost all electronic equipment, remove EFT’s.

what is the basic difference betwwen real,active and apparent power and what is the relation between them
Trying to get a better understanding of real vs. apparent power. Reading the paragraph on apparent power, the indication is apparent power is actually delivered to the load. Question for me remains whether that apparent power should be considered as “consumed” from the perspective of the power source. Would help me to better understand if I can get clarification on the following example I need to select an UPS and know both apparent and real power values for the system being powered. Ultimately I want to ensure the UPS will provide sufficient back up power for a duration required. Will apparent power have a direct impact on the available reserves in that UPS source to power the system.
Apparant Power is what is delivered by the energy source which is greater than the work done by the device (Real Power)
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this is really great explanation.Thanks alot.