I saw an article on the web several years ago describing how to convert an AC microwave to twelve volts. I have tried to find it but have been unable to. If any one has seen it and remembers where it is I would greatly appreciate if he would post it here. Failing that I would have to start from scratch. I don't think it would take much more than rewinding the primary of the transformer and a simple FET drive. The microwave I have in mind uses a simple dial timer. I would leave the timer and turntable on AC and have the output of the timer drive a relay for the DC. The advantage of this is to reduce the losses from an inverter. Even for a 600 watt microwave the loss through even an efficient inverter is not negligible. The microwave will actually use about 850 watts. The amperage (around 80) should be within the range of some readily available FETs.