Microwaves normally have an “ouput power” rating like 500-1000W (watts). Many people incorrectly assume that this is the power that the appliance consumes. Typically, microwave will draw 40% more. This gets worse when running off an inverter due to their poor power factor.
E.g. an 800W rated microwave, consumes about 1600W of 12v power through an inverter – ie DOUBLE!
When calculating inverter size and battery requirements, use a factor of 2x the “rated” wattage of your microwave!