What you describe is a fairly common design.
A solution to "wasting" electric power to heat the first (main) HW tank
when heated water is available in the storage tanks is to add a
controller and some plumbing (and a small pump) that will sense when the
main HW tank is cold and pump heated water in from the storage tanks.
To do this one needs a fairly standard differential thermostat type
controller and sensors, a small (1/10hp) pump and some additional
plumbing between the tanks (primarily a return line from the main HW
tank to the storage tank, pump to go in this return line). You set the
temp on this new controller to turn on a bit lower than the temp setting
of the HW heater (and turn off a bit higher). This will keep your main
HW tank hot as long as there is solar HW to do so. The electric will
still function as a backup, and no risk of distributing electrically
heated HW to the solar panels...
If you are doing this yourself, it is a fairly simple/cheap change. If
you will be paying someone to do it, it may be more cost effective to
replace the HW tank heater with a flash type one...
A bronze pump would be required, which are quite expensive. Then gthere is
teh payback of buting the punmpa dncopnteroller. After all water entering
the main tank will be already heated by the solar pre-heat tank. The only
inefficient aspect is that standing heat losses may degrade the heat in the
What is flash type water heater/tank?