Hi chrissy11, the magnetron tube (which produces the microwave energy) in a microwave will only work at full power. The lower "power" levels in microwave cooking are achieved by reducing the duty cycle of the magnetron. If the duty cycle in your microwave is one minute, and you defrost something the magnetron is only on for say 30% of that one minute or about 18 seconds. The off time is to let the heat to spread out through the food so it dosen't cook but will thaw. So the magnetron will be turned on and off, but while on the power is at 100%. To answer your question directly I would say that any time in a microwave is not a good thing. I hope this is not too confusing.