Good indefatigable, where are you from? I am from the central area of Spain and we have dealt with some technicians because we wanted to find out about some materials for a "new" work, so to speak, and we contacted Tauleria Almenar, a company specialized in the manufacture and sale of roof tiles...
It is difficult to determine if an off-grid system will work without more specific information about the system in question. Factors such as location, energy usage, and system components all play a role in the effectiveness of an off-grid system.
Hello Bigbillsd, is there a specific reason why you are considering this strategy of gradually installing solar panels on a grid tie system? Do you have any budget limits or physical space in which you can install the solar panels?
If it helps you to calculate or have an idea...