In our October blog, we suggested that you err on the side of larger when sizing solar panels and batteries. One reason is that solar panel batteries are greatly affected by temperature; and only one part of this story is good.
Battery charging voltage also changes with temperature. It will vary from about 2.74 volts per cell (16.4 volts) at -40 F to 2.3 volts per cell (13.8 volts) at 122 F. This is why you should have temperature compensation on your charger or charge control if your batteries are outside and/or subject to wide temperature variations. Some charge controls have temperature compensation built in (such as Morningstar). This works fine if the controller is subject to the same temperatures as the batteries.
Even though battery capacity at high temperatures is higher, battery life is shortened. Battery life is reduced at higher temperatures. For every 15 degrees F over 77, battery life is cut in half. This holds true for ANY type of Lead-Acid battery, whether sealed, gelled, AGM, industrial or whatever. This is actually not as bad as it seems, as the battery will tend to average out the good and bad times.
To combat the issue, place the battery in a well-ventilated enclosure; preferably white in color. Try to position the enclosure out of direct sunlight whenever possible.