Background: Late in the summer last year I finally commissioned a DIY solar hot water pre-heat project. Very simple, drain back, open (vented) tank, PEX coil for a heat exchanger, PEX plumbing ...
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: ...