Heat retention with a solar cover is all about stopping evaporation. Like blowing on your coffee or soup to cool it off. You don't HAVE to cover every square inch, but the more area you cover, the less evaporation and less heat escapes. Cover also helps keep stuff from falling in the pool (leaves & pollen, etc). Definetly worth the trouble in areas that struggle to keep the pool warm enough. To help put cover on & off, I attached a long piece of nylon webbing, like the strapping on a backpack. This helps to pull the cover on with one person. A reel is great for taking the cover off and storing it while the pool is in use.