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solar storage, one tank or two

? I have an existing solar hot water system.  I have a separate shed built to house a 300 gallon non-pressureized drain back tank and 280 square feet of collector area.  In the house I have an electric water heater- which provides the DHW and heat (through a flat plate heat exchanger) to cast iron radiators.  I want to increase my efficiency of delivering solar heated and pre-heated water to the the water heater in the house.  I am thinking to use a 120 gallon electric water heater with just the top elements on low as backup.  But I am concerned about overly mixing the water in the tank, and not maintaining stratification of the water (hotest on top).

A friend with a lot of solar experience suggested 2 separate tanks, the electic back up tank being small, 20-30 gallons, and having two 5500 W elements for fast recovery.  The idea is to have more control over mixing,to maintain stratifaication and not to mix electrcally -heated water with the solar heated water, unless the solar heated water is hotter than the back up heated water.  It would accomplish what a on demand water heater does, minimum storage electrically-heated water, but ability to deliver a lot when the demand is high.

Do you know about systems that use these concepts? Or should I just use the 120 gal tank?
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