This is a test of commonsense. Complete the next sentence:

How to create a solar panel water heater
Get a double-glazed unit for the sunlight to pass through.
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.
Get a single-panel room radiator that is slightly smaller than the glass unit. Scrap yards sometimes have old radiators, or ask a plumber who is refurbishing a central heating system.