Re: Best solar pool cover for heating up pool and retaining
"The cover independent of whether it lets light through will cut down carbon dioxide outgassing so will reduce the tendency of the pH to rise if the TA is already high enough for that to be happening in your pool. With an opaque cover, you eliminate the breakdown of chlorine from the UV in sunlight. So you should see a far lower chlorine loss for the days you have the pool covered (sound like just the weekend days in your case). There should be no change with regard to any other chemicals or water parameters (except temperature, as noted below).
As for whether you will get any additional net heat gain, this depends on the color of the cover and the amount of evaporation in your pool. A very dark cover will add some heat to the pool during the day while a light or reflective cover will not add any noticeable heat. The cover will help retain heat, especially at night.
I have a mostly opaque pool cover on my pool and it is on all the time except 1-2 hours every day when the pool is used for swimming (somewhat longer on the weekends). The main effects are to reduce the amount of temperature loss at night and to reduce chlorine loss during the day. You will benefit from the former, but not so much from the latter except on the weekends."
The above is text I copied on this site-can't remember the author. Hope it helps.