Since the solar panels only produce electricity in daylight (direct sunlight is unecessary), you also need a means to store the electricity to keep the water hot at night. The capacity that you will need varies according to the length of the day, and therefore your location.
We are big fans now of lithium. True they look expensive, but they tolerate a daily discharge rate of 50% and 80% occasionally. That compares with 20% for deep discharge AGM lead/acid batteries. That means you always need 2 1/2 times as much AGM as lithium and four times may not be far from the mark. Lithium's active life is also 2 or 3 times greater. Despite the higher initial cost, we think it is a no-brainer!