Formula for calculation of efficiency

General
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Understanding the efficiency formula for a 3-phase 3L NPC inverter

Hello,

I am currently simulating a 3-phase 3-level NPC inverter and checking its efficiency using different combinations of IGBTs and diodes. The initial model comes from a confidential source and uses the following formula to calculate efficiency:

\text{Efficiency} = \left(1 - \frac{P_\text{loss}}{P_\text{out}}\right) \times 100

Where:

  • Pout is the output power of the inverter (calculated from its nominal value)
  • Ploss is the total switching loss of the IGBTs and diodes

What I don’t understand is why this formula uses Pout in the denominator. Intuitively, it seems more correct to use:

\text{Efficiency} = \left(1 - \frac{P_\text{loss}}{P_\text{in}}\right) \times 100

where Pin is the input power.

After analyzing, I realized that the original formula is approximately valid if Ploss << Pout, which is true in my simulations. However, I could not find any references confirming whether this is an industry standard.

Questions:

  1. Is using Pout in the denominator a common practice in inverter efficiency calculations?
  2. Under what conditions is this formula considered accurate?

Any insights or references would be greatly appreciated!