The efficiency of magnetron is about 73%.

This can be tested according to the national standard GB/T 18800IDTEC60705 "Performance Test Method for Household Microwave Ovens".

Simple method:

Fill a glass with 1000cc of water, and control the initial temperature at about 10 degree;

Let the microwave oven work t=4 1870/ rated output power of the microwave oven (seconds);

Then measure the heated water temperature t and calculate the temperature difference △ t before and after heating;

Actual output power =△T* rated output power/10