Whenever your pool water is heated (whether from the sun via a Solar Panel System, from the air via a Heat Pump, or by Electric, Oil or Gas Heaters), your pool will also lose heat, which will make the heat up time longer, and obviously take more power and cost.
Did you know that around 90% of the lost heat is from the pool water's surface. As we all know, heat rises, but also the air above the water can cause evaporation, chilling the water. Therefore, the first thing to do is reduce the evaporation by creating a barrier covering the pool water, usually with a plastic sheet. You can do even better than this with a plastic bubble sheet. Each of the bubbles on the summer covers (usually around 1 cm diameter each) acts like a tiny greenhouse to take the solar heat from the sun and transfer it to the pool water below.
Obviously, the thicker the sheet, the better and longer lasting (especially important in our British Climate), and you can go one step further by giving a metallic reflective surface on the underside of the cover to hold more heat in - imagine putting silver foil at the back of your household radiators. The principle is the same, the heat going in the wrong direction is reflected back to where you want the heat, back into the swimming pool water.
Traditional solar covers have used round bubbles on the surface but this new cover, proven to be more effective, has peanut shaped bubbles which are better at heating as they have a larger area for air expansion and also have been designed to combat the weak points associated with the round design and to combat UV attacks which destrouy the cover's stability. The actual material is also thicker with the stresspoints, like the corners, reinforced to prolong the life of the cover.
You will also find that when your pump is running, your return jet aiming downwards will circulate the warmer water from just below the cover to mix with the cooler water below.