The 'what if the sun don't shine' problem is the difficult issue in solar thermal. Where can I find good solid test-based resources on the options for backup and their downsides? For example:
Single tank with gas backup: gas standby losses, heater kicks in even if water is cold but there is no demand, minimum flow rate too high.
Single tank with electric backup: heater kicks in even if water is cold but there is no demand.
Tankless gas: high cost, inconsistent output temperature, gas service upgrades.
Tankless electric: electric service upgrades, higher per-btu cost.
2nd tank gas: high standby losses.
2nd tank electric: higher per-btu cost, but cheap to buy and effective?
Where are the good resources on this topic? Does the answer change if a small radiant heat loop is in the mix?