[header] How to create a solar panel water heater [title] Get a double-glazed unit for the sunlight to pass through. [step] Glaziers and window manufacturers sometimes have scrap units for patio doors or large windows that can be bought very cheaply. Glass with a low-emissivity coating such as pilkington k glass is ideal. 
[title] Get a single-panel room radiator that is slightly smaller than the glass unit. [step] Scrap yards sometimes have old radiators, or ask a plumber who is refurbishing a central heating system.