Microwaves do not have a heating element. The cooking energy is generated by a magnetron. Convection ovens do have a heating element. If you have a microwave/convection oven combo then it has both.
A heating element is a resistance heating device. It uses exactly the same amount of electricity at start up as it does any other time. Most of the larger heating element type appliances will use at most 1,500 watts on a high setting. Some smaller toasters and coffee makers will run about 800 watts.
1,500 watts from an electric skillet for instance is about 150 amps at 12 volt DC. You would want to have around 500 Ah of battery capacity as an absolute minimum to try and run a load like that. Otherwise you're really not going to get much life from your batteries.
A microwave using about 800 to 1,000 watts would still be best served by that size of bank as a minimum.
