Where can I find good resources on solar thermal backup options?
For me with a solar system in New Zealand, we've found the following quite effective: we've got a solar tank with 2 electrical elements in it, one at the bottom of the tank, plus another one about a third of the way down from the top.

We run the bottom element overnight intermittently during the winter months when the whole tank is cool, the forecast is poor, and we know we'll need hot water the next day.

We have the top element on a separate booster switch which, on the rare occasion that we want a shower and we can see from the control panel that the water at the top of the tank isn't hot enough, is set up to heat the top of the tank; when the thermostat drops out as the water gets hot enough, the booster switch turns itself off, so we don't have to remember.