Originally Posted by Puddin'_Tain
Look at the current draw. Even a small micro way will require more current than a small boat can easily provide. Almost all compact microwaves call for a 15A circuit. Even if you can get away with 10A, that's 10A at 120V, or 1200W. In other words, the draw on your boat's batteries would be about 100A at 12.5V. On big boats, with big battery banks and gensets, that's doable. On a 27-foot boat, not likely.
If a microwave is used sparingly, as they nearly always are, the DC amp usage is pretty low. An inverter is needed. But your 1200 watt example need not be that high. A small microwave at 700 watts AC will use about 70 amps DC through the inverter. 5 minutes of use to heat something up or make popcorn or whatever will use 70 divided by 12 or a bit less than 6 amps DC.
I have a 700 watt microwave on my 27' boat.