In the field of electrical engineering, a pu is a unit of apparent power that is equal to the product of the rootmeansquare (RMS) voltage and the RMS current of an alternating current circuit. Voltamperes is the unit of measurement for it (VA).
What is “pu”?
This “pu” will be used in a single line diagram of power distribution, and it is similar to a huge electrical circuit with a number of components  generators, transformers, loads with different ratings. The abbreviation “pu” stands for “per unit,” and it will be used in this diagram (in MVA and KV). We use the pu concept to bring all of the ratings onto a unified platform. According to this concept, the component with the highest MVA and KV ratings is used to determine the base values, and then all of the other component ratings are recalculated using these values as their starting point.
Formula
p.u value = actual value/base value
Where,
Actual Value = the unscaled quantity in appropriate SI units and a phasor or complex number in an alternating current circuit; and, where applicable, a time function.
Base Value = a real number chosen to satisfy the scaling axioms & achieve as many desirable properties as possible.
PerUnit Value = the scaled dimensionless quantity and a phasor of complex number in an alternating current circuit situation; and, when appropriate, a function of time.
Calculations that involve huge units can be made more simple by using this method.
A number of units, often power and voltage, will be chosen as bases, and all of the other units’ calculations for each unit will be predicated.
Example
Assume that a threephase power system that can generate 500 megawatts at 138 kilovolts.
Power (pu) = 500 MW = 1 pu
Voltage (pu) = 138 kV = 1 pu
Current (pu) = 500 MW / (138 kV X √ 3 ) = 2.09 kA = 1 pu
Impedance (pu) = 138 kV / (2.09 kA X √ 3 ) = 38.1 Ohms = 1 pu
Other units can be derived as needed.
A reading of 136 kV is obtained by taking a voltage reading at a selected point inside the system.
On per unit basis,
V1 = 136 kV/138 kV = 0.98 pu
A schematic representation of the system that includes the actual voltages expressed in
V1 = 0.98 pu
V2 = 0.93 pu
V3 = 0.91 pu
Using the per unit system, which distribution points is determines as fall below the system specification of Voltage delivered > 0.91 pu.
Advantages OF Per Unit

A clear understanding of relative magnitudes

More comparable voltage levels

Maintain the same order of voltage drops and losses.

Calculations that are as simple as possible

The calculation of the transformer turns ratio has been simplified.

Handling a small number