The PET bottle lights are not my invention – they have been used in the Philippines for many years as a very cheap alternative to electric lights in a corrugated steel roof. They are siliconed in to the roof to make them watertight and are approximately 50-60W – most adequate in dark locations and with no running costs. They work in full sunlight and even on cloudy days and are an excellent renewable light source, using a throw-away product.
One of my lights has water in – the other is just air – with hardly any difference in luminosity.
A difficult photographic challenge !