Solar panels are rated much like air conditioners, in BTU's per square foot. The more heat that is put out, the better the BTU's will be. Normal BTU's for solar panels are 900 to 1000 output. There is the small difference according into the brand which can be used. Choosing the right size panels are usually essential for maintaining water temperatures at the low to mid 80's, and might be calculated according to size. A satisfactory system always be at least 50 to 100 percent of pool surface type. Panels come in 4x8's, 4x10's and 4x12's, although custom sizes can usually be used.