In reference to schematics like the first one on this page:

Why do schematics like this always show the backup heater in line with the output of the solar heater? Wouldn't you then always be using your electrically-heated water first? It makes more sense to me to install an electric heater in parallel with the solar heater, with isolation valves so you could run the backup heater only when needed.

If it makes a difference, I live in Tucson, AZ. I know people around here with solar HW systems with no backup heater at all.