Question

why we use average power average power instead of total power

Answer 1

Electrical Power (PP) as we know is the product of voltage and current (VIVI).

But is it the product of instantaneous voltage and current or average voltage and current or to make it even worse is it the product of RMS voltage and current.

Lets get to the basic definitions of Power.

Instantaneous Power (PiPi) = Vi∗IiVi∗Ii ;

This information is very fundamental,because the concept of Average and RMS is derived from this basic idea by us to explain certain behavior that happen in electric circuits. The very interesting part here is, the devices that we work with (be it a resistor, or a battery) doesn't know what is the average or RMS values it carry. In very strict sense we could say that the circuit elements only responds to instantaneous values of voltage and current, but on the whole the system (or that element in particular) may exhibit certain behavior, that could only be explained using mathematical tools like average and RMS.

Average Power

Lets start to analyse it,by taking the definition of Average Power(PaPa)

The concept of Average power is observed almost everywhere in Electrical Engineering;The applications of P_avg are many, to list a few includes:

Energy meter reading (Energy is an integration function of PaPa)Regenerative breaking (as in line com-mutated Inverter, the direction of power flow depends of the sign of PaPa)Direction of flow of power in Electrical Power system and Load-frequency control.Heating of LoadsCharging of Battery

To be frank this is the most important (may be only) Power we deal with.

P_avg = {Integral over 0 to T of (P_inst)} / T (Sorry about the Math expression!)

Lets take 3 cases to explain this concept

Case 1: AC Supply - where both V_inst and I_inst are assumed to be pure sinusoidal, with phase shifts corresponding to the type of load it is connected to (Lead or Lag) (Note only RMS values exists here and Avg values are zero)

Here after performing the mathematical derivations(as per definitions of P_a)

Pa=VR∗IR∗Cos(phi)Pa=VR∗IR∗Cos(phi)

where phi is the angle between V_inst and I_inst, VRVR and IRIR are the RMS values of V and I respectively.

Case 2: An Output from a Single Phase Diode rectifier circuit - where there exists both Average and RMS values of V and I

Here let me take 2 examples of power consumption,

Heating of Resistor loadCharging of a Cell

What’s interesting here, is the expression for PaPa takes different forms for both examples.

In a resistor, Power is consumed as heat(this is the physical behavior we observe).

Here Pa=VR∗IRPa=VR∗IR (remember UPF Load!).

why RMS values,recalls us to the definition of RMS Current (it is that value of of a waveform that produces the same amount of heating in a resistor as done by a DC current of same magnitude for the same amount of time)

In a Cell, Power is consumed in chargingthe cell (this is the physical behavior we observe)

This is where the definition of Average current comes to help (it is that value of a waveform that produces the same amount of charge as that of a DC current of same magnitude for a specified time)

So Pa=Va∗IaPa=Va∗Ia

Case 3: DC - where RMS values = Avg Values = DC Steady State Values

Here Pa=Va∗IaPa=Va∗Ia (Also called DC Power)

Now that’s the expression of P_avg, a physical behavior we observe in electrical circuits.P_avg is also referred as Real Power or Active Power.

That’s all about of Average Power in a nut shell.

RMS power is only having a mathematical expression which is the Root Mean Square of Pi(=Vi∗Ii)Pi(=Vi∗Ii),and has no physical significance (as far as I know!)

And AC Power in general refers to Complex Power VI(∗)VI(∗) [V x I’s Conjugate], which is a general expression that includes both Average (or Active) and Reactive Power.

As Real {Complex Power} = P_a; and Im {Complex power} = Reactive Power.

I hope this answer can give you an overview of Electrical Power in general.

Answer 2

Answer:

It is the average amount of work done or energy converted per unit of time. The average power is often simply called "power" when the context makes it clear.