What is the cause of the tractor output?
There is a large margin of error (interruption of switching power supply) when the current is measured using a normal ammeter (such as a multimeter). The power you calculated on the ac side should be seen at power, which is 21.1 VA. The real power (power) should be the power factor. The power factor can be reduced to about 0.5 because of the power factor correction circuit without power factor.
It is thought that The switching power supply has a standard called efficiency. The average is below 0.75. Low power switching power supply is between 0.6 and 0.7.
So, the input power is 21.1 * 0.5 = 10.55 W, and then 10.55 * 0.6 = 6.3 W, which is not much difference. If you want to measure the correct output power, you can only use the power meter.
