We use our microwave for two purposes. Reheating and....... as a breadbox. Seriously, store bread in the micro and it will last substantially longer than on the counter. Dark and no air circulation. Easy to pull out when you need to use the oven
As far as wattage, it seems you would need to have one with lower wattage output to even be able to handle an exception on many invertors. I suppose it depends on your invertor output and house bank capacity. However, lower wattage take much longer to cook/heat, so I'm not sure you really gain any ground. Unfortunately, micros and house batteries are as compatible as nuts and gum.
Personally, I would get the highest wattage unit and understand it lives off shore power and the genset. Basic timer is about all you really need, all the silly preprogrammed buttons are ridiculous. I don't need a button that says, potato or popcorn.