Heat loss due to evaporation is the biggest heat loss of a pool, an order of magnitude greater than convective losses. I live in NJ and keep the solar cover on whenever we are not using the pool - it makes a huge difference. I run my heatpump to initially raise the temperature in the early spring - that is it. I have two solar mats that are 2' by 20' each, and between them and the cover my pool stays at 80-85 degrees all the time.