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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?

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This site could be a good resource, if you asked the right question. –  Tester101 Dec 5 '12 at 12:31
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1 Answer

up vote 2 down vote accepted

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.

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