For year-round outdoor pools, approximately 50-60% of the surface area of the pool should be matched with equivalent Gobi collector area. In many Southern states (depending upon hours of sunlight) that percentage can often be dropped to 35% area. Sizing for indoor pools is a little trickier, as they don't get any solar "gain" or direct sun heating them all day. Typically for indoor pools, 50-60% of the surface area in equivalent Gobi collector area will suffice. Areas that get multiple or severe freezes require a closed-loop glycol system with a heat exchanger.
It is always highly recommended to use a pool blanket or cover for the pool to keep the heat in and stop the pool from cooling down overnight.