It isn't possible to vary the power in a microwave oven. It's either on or off. The "power" setting works by controlling the duty cycle, what percentage of the time it's on and it's off.That used to be true, however newer ovens have (supposedly) variable-power control, for instance the Whirlpool Microvens with AccuWaveâ„¢.
posted by inigo2 at 11:12 AM on July 27, 2006