I am looking for the convertor unit that takes 12 volt DC and converts it to 120 AC and also hooks to the 120 volt power grid of the electric company. If I am using less than I am generating by solar it supplements from the electric company. If I am generating more than I am using it feeds the excess into the electric company power grid.

These type of converters are not off the shelf type or DIY units. You need to inquire about this with your pubic utility company and with a licensed professional electrical contractor who does this type of work.

If incorrectly installed you can cause an electrical hazard to any lineman working on the power grid if its turned off for service and their not knowing your still on line.