It’s noon and bright outside, yet the back room is dim enough that you reach for the switch. The only window faces a wall, and the overhead light stays on most of the day. You’ve thought about a skylight, then pictured the cost and the leak risk.
This is not your fault. A room without a good window simply has no way to let daylight in. A Brazilian mechanic found a much simpler fix, with an empty 2-liter plastic bottle.
How a bottle of water turns into a light
Alfredo Moser started using bottle lights in 2002 in Uberaba, Brazil, during an energy shortage when frequent blackouts left only factories with power. His explanation of how it works is short: “Simple refraction of sunlight.” The bottle sits in a hole in the roof, cap outside. Sunlight hits the part above the roof, the water bends it, and the part below lights up the room.
The bleach is there for the water, not the light. Moser adds two capfuls so the water doesn’t turn green with algae. As for brightness, he puts it simply: “It depends on how strong the sun is but it’s more or less 40 to 60 watts.”
The idea traveled far. In April 2011, Illac Diaz launched Liter of Light through the MyShelter Foundation in the Philippines. By 2013, more than 140,000 homes in the Philippines, plus homes in 15 other countries, reportedly had Moser lights, and Diaz estimated that one million people would benefit from them.
The build, in the order the guide gives it
Liter of Light’s step-by-step instructions, shared by the UN refugee agency’s innovation team, are built around a corrugated metal roof:
- Make the collar. Cut a 10 x 10 inch piece of corrugated galvanized iron roofing. On top of a ridge, draw a circle the size of a large clear plastic bottle, and a second one 1 cm (about 0.4 inch) smaller inside it.
- Cut the tabs. Cut out the inner circle, then cut lines out to the outer circle and bend the tabs up at 90 degrees.
- Lock the bottle in. Scratch the bottle about a third of the way down from the top, push it through until the tabs meet the scratched band, and seal the joint generously above and below. Let the sealant dry thoroughly.
- Fill it. Add 10 ml of bleach, about 2 teaspoons, fill the bottle completely with water and screw the cap on tight.
- Mount it on the roof. Cut a bottle-sized hole, seal around it, set the collar in with the cap facing up, rivet it with 4 rivets on each side, then seal every edge and rivet. Finish with a protective plastic tube over the cap, covered in sealant.
Plan B if you don’t want to cut a roof: try the principle on a small shed or a backyard project first, where a leak costs a lot less than one above a bedroom.
Where the bottle bulb falls short
It only works in daylight. Moser’s own estimate depends on how strong the sun is, so a cloudy afternoon gives less, and at night it gives nothing.
The roof is the real risk. The guide’s method relies on thin corrugated metal sheets, rivets and plenty of sealant. A typical shingled roof with an attic underneath is a different structure, and a bottle there would mostly light the attic. Any hole in a roof can leak if the seal fails, so if you’re unsure, call a roofer.
For a dark room in a standard house, the store-bought cousin of this idea is a tubular daylighting device, a small tube skylight. ENERGY STAR includes residential skylights and tubular daylighting devices in its Most Efficient program.
Water, a splash of bleach, a well-sealed roof and full sun. That’s the trick.
