I've read alot of helpful info on this site! We live in Chicago. I am starting the remodel of our breezeway, kitchen and converting part of our garage into living space. About 550 sq ft. I would love to do radiant flooring. I had two people tell me electric would be better. And one tell me I would need supplemental heating, radiant would not be enough. We would be putting tile in all. All these areas are on a concrete slab and I can not tell you how cold that is! Right now we have forced air in the main part of the house and two wall gas heaters in the part we are remodeling. I read electric can be more expensive to run but in a year or two I would like to install some solar panels. We are starting in two weeks and need to get this decided. Our only other option would be to raise the floor some and stay with the wall heaters which I hate.
If you want the system to be a primary source of heat, you will need to either supply the heat loads so that you know how much heat you need or have the company who supplies it complete a Heat Loss Calculation. That is the single most important step before looking into any radiant system.
That said, a low voltage electric can be used as a primary heat. We have several clients who have done or are in the middle of doing exactly what you plan regarding solar panels. I wouldn't plan on them being able to supply all the power you require but they definilty make a big dent in it.
What is the constructoion design? insulation/plywood/concrete, walls,floors, what type of finished floor, etc..