The solar collector area is the flat panel area of the solar hot water system that is exposed to the sun.
For multi unit developments an approximate rule of thumb is that 0.5 – 1 flat plate panel (2m2) will usually be required per apartment.
Note: Solar collectors seem to have a smaller area when displayed in plan view, since they are typically installed at an angle to catch the sun, and not installed horizontally. The solar collector area entered in BASIX should be calculated based on the panel dimensions from the manufacturer, and not straight from the plans.
Contact your hot water system manufacturer for the best advice on sizing a large scale solar hot water system for your proposed development.