Legionnaires' disease with SHW system

Hey Ben, as Amy said, most pasteurization of water borne bacteria is a time -temperature relationship. Messing with temperature settings would be more of a safety concern to  me, unless you drink a lot of hot tap water.
  I really don't like having the electric back up in a solar tank since they tend to compete with each other. Normal setting for most any tank is 120-130 deg. so by time it gets to a faucet it is down to 110. Shower valves are to be set at this temperature too, so irregardless of incoming hot water temp. you can't burn yourself in the shower. 
   The thing with electric backup, if you use hot water at night, then the electric kicks in. Then in the morning, when sun is available, and depending where your sensor is located on the tank, the solar will stay off until it gets up to well over the already hot, tank water temperature. This adds lots of energy real quick and can raise the top of the tank temperature well over safety levels, This is great to keep the electricity off but you may notice your relief valve on the tank start to drip. 
    If your sensor is located near the bottom of the tank, and is set for a high limit of 160, then the solar will keep heating and possible bring top of tank temperatures near boiling. 
  If you are getting plenty of sun and use minimal hot water, I would just shut the electric off all together and locate the sensor properly on the tank and use your hot water wisely
. If you know you need hot water and it is cloudy, it is easy enough to turn on the electricity a few minutes before you need it.
  If pathogens are that much of a concern,  a UV light type device may be your answer.