i have a 40 foot oval with a gas heater. i use the solar cover always. mostly as max said, to hold the heat in over night. if i dont put the cover on it steams at night . brings tears to my eyes as that cloud slowly rises over the clear blue water. looks like dollar bills just floating away. i tried the cover for a week or two when i first got the pool. what a pain in the a@# . you will not want to wrestle a 21 x41 solar cover on and off that pool. it is so easy with the reel . i do it myself most of the time . just roll it up and pivot it to one side. no grass from pulling it off and floppin it on the ground. its a must have.....just my opinion. ron