This is a big variable as it will depend on where you are located, how insulated is your house is and how many square feet. When heating with solar you will also need to decide how much storage you want as there will be days that the sun does not shine as well as evening demand when there is zero sun. On average we recommend anywhere from 200-400 square feet for each collector. The storage will also very but the more storage, the better the system will perform.
Homes or shops that use in floor heat in concrete, can use the concrete as a storage means however they will lack some heat control as the floor will heat up during the day and loose the heat at night. For shop and garages this is a great and affordable way of space heating as there is no need for a large storage tank.
During the summer months a homeowner will need to cover up the collectors as there will be no demand unless they also happen to own a pool. If they have a pool, then the extra heat in the summer can be used to heat the pool.