The solar-powered pump might struggle to raise the water to a high roof roof for a drainback system. I don't know if a sealed & presurised system might be easier or harder for the pump.
For my system though it is an interesting option. My panels are on the wall, not the roof. The total head in the system is less than 2 metres with the panels half way up that height. A relatively weak pump might cope with that.
I don't know why I didn't think of it when I installed my system but it might be a worthwhile item to retrofit as a backup in case of power failure on a sunny day. If it worked my mains-powered pump could be configured to kick in at a higher temperature delta so that normal pumping operations would be done by the DC pump and the AC pump would only kick in in extremis. I will give that some thought.
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