a pool cover having solar energy heating capability is provided comprising sheet material adapted to furl about a reelless locus in a storage position and to be deployed in an extended position . the cover includes means continuously resiliently urging the sheet material into its furled position for storage , and means selectively overcoming and resisting said urging , comprising inflatable structure selectively inflated in sheet material deploying relation . the cover includes a wall portion responsive to solar energy to transfer heat to water , such as pool water , in contact therewith .