Small RV Microwave

That is how a standard microwave works, the power level just dictates the ratio of on vs off time of the magnetron. An "Inverter" model will actually reduce the energy with power level. You can also find lower power standard units. The watts printed on the box are just the "cooking power" It takes power for the rotating plate, the light, the control board. Also, a microwave is not just a resistive load, they usually have a poor power factor and thus create a higher apparent load to the device providing the power.