This is a cut and paste from: http://www.fsec.ucf.edu/En/consumer/solar_hot_water/pools/q_and_a/index.htm#Cover
Q: What about pool covers?
The greatest loss of heat from a pool occurs from its surface because of evaporation. By reducing this evaporation loss, pool covers are very effective in lengthening the swimming season. They also keep the pool clean, thereby lowering the cost of chemicals and filter maintenance. Depending on materials and the number of hours of use, temperature increases of 5°F to 10°F may be expected from a pool cover. A 5°F increase is reasonable when the cover is used 12 hours a day; 10°F when it is used 20 hours a day.
Transparent or lightly translucent covers work best because they allow solar energy to pass through and be absorbed by the pool water, and they also prevent heat loss at night. Opaque covers are best used in Florida at night to prevent heat loss. A roller is a good investment to help you move the cover on and off the pool. Motorized rollers are also available.
Pool covers will last from three to five years, depending on care in handling and storage. Nevertheless, they are your best buy for an extended swimming season. From the standpoint of energy conservation, a pool cover should be used.
Q: What about pool covers?
The greatest loss of heat from a pool occurs from its surface because of evaporation. By reducing this evaporation loss, pool covers are very effective in lengthening the swimming season. They also keep the pool clean, thereby lowering the cost of chemicals and filter maintenance. Depending on materials and the number of hours of use, temperature increases of 5°F to 10°F may be expected from a pool cover. A 5°F increase is reasonable when the cover is used 12 hours a day; 10°F when it is used 20 hours a day.
Transparent or lightly translucent covers work best because they allow solar energy to pass through and be absorbed by the pool water, and they also prevent heat loss at night. Opaque covers are best used in Florida at night to prevent heat loss. A roller is a good investment to help you move the cover on and off the pool. Motorized rollers are also available.
Pool covers will last from three to five years, depending on care in handling and storage. Nevertheless, they are your best buy for an extended swimming season. From the standpoint of energy conservation, a pool cover should be used.