How do you calculate the multiplication factor of energy meters?

Look for the ct (current transformer). You will find a ratio rating (for example, 200:5.) This means divide 200 by 5. The result is 40. Any difference in reading from a certain time should be multiplied by 40 to get actual energy consumption. This is the basic information, but in practice it should be calibrated by a government-certified body (the department of energy, for example) to perfectly match the kwhr-meter revolution.