Portable spas are usually heated with electricity... which is no longer cheap! Furthermore, they take a day or more to heat, so they must be kept heated all the time. A solar spa heater is a very cost effective system, with complete payback in around 5 years. The usual collector area is less than twice the area of the spa... because they are well insulated. A small pump moves the spa water up through the glazed panel(s) to absorb the heat of the sun.
In-ground spas are not well insulated, and therefore require larger systems. They are usually gas heated, and heat very quickly (typically 30 minutes). Solar only makes sense on in-ground spas if you plan to use it almost nightly.