A builder friend of mine knocked up some diy solar panels, he ripped some large flat panel rads out of a school, installed them in like a cold frame on top of encapsulated insulation that and the rads were painted black, rads had 3/4 tappings so 22 mm pipe gravity feed to an indirect fortic at high spot in the roof, that fed the C/F to the H/W cyl in the airing cupboard, no electrics, never went wrong free hot water from April to Oct