A solar still is a simple device that uses sunlight to purify water through evaporation and condensation. It works the same way rain does: heat turns water into vapor, leaving contaminants behind, and the vapor condenses back into clean liquid. Solar stills can turn saltwater, brackish water, or polluted water into safe drinking water without electricity, chemicals, or moving parts.
How a Solar Still Works
The process inside a solar still mirrors Earth’s water cycle in miniature. Sunlight passes through a transparent cover, usually glass or clear plastic, and strikes a dark-colored basin filled with contaminated water. The dark surface absorbs the sun’s energy and heats the water, speeding up evaporation. As the water turns to vapor, it rises and meets the cooler underside of the glass cover. There, it condenses into droplets, just like moisture forming on the outside of a cold glass on a hot day. Those droplets slide down the angled cover, driven by gravity, into a collection trough at the bottom edge. The collected water is distilled and free of salt, heavy metals, bacteria, and most other impurities.
What makes this effective is that only the water molecules evaporate. Dissolved salts, sediment, pathogens, and organic waste stay behind in the basin. Testing on real seawater shows salt removal rates of 99.99%. For various types of wastewater, including municipal sewage, textile runoff, and kitchen wastewater, organic contaminant removal ranges from about 86% to 98%.
What Solar Stills Can and Cannot Remove
Solar distillation handles salts, bacteria, heavy metals, and most dissolved solids extremely well. It is one of the few purification methods that can make seawater drinkable using no energy source other than the sun. It also effectively reduces the chemical oxygen demand of heavily polluted water, meaning it strips out the bulk of organic contamination.
The notable weakness is volatile organic compounds. These are chemicals with low boiling points, things like certain pesticides, solvents, and industrial pollutants, that evaporate right along with the water and end up in the distilled output. A standard solar still has no mechanism to separate these from the water vapor. Researchers have developed experimental solutions using photocatalytic materials inside the evaporator that break down volatile organics before they reach the condensation surface, but these are not part of a typical DIY or commercial solar still. If you suspect your water source contains industrial solvents or pesticides, a basic solar still alone may not produce safe water.
Typical Water Output
A conventional single-basin solar still produces between 0.5 and 7 liters of clean water per square meter of basin area per day. The wide range depends on sunlight intensity, ambient temperature, design quality, and materials. In moderate climates, expect roughly 2 to 3 liters per square meter daily. In regions with strong solar radiation (4 to 8 kilowatt-hours per square meter per day), output can reach 5 to 7 liters.
That output is enough for emergency survival or supplemental drinking water, but it puts practical limits on the technology. A single square meter of still won’t meet a household’s full daily water needs. Scaling up means building a larger basin or multiple units, which is feasible in rural settings where land and sunlight are abundant but impractical in dense urban areas.
Design Details That Affect Performance
Three design choices have an outsized impact on how much water a solar still produces: the basin liner material, the glass cover angle, and the water depth.
The basin liner is the dark surface that absorbs sunlight and transfers heat to the water. Synthetic graphite performs best among commonly tested materials, achieving about 35% efficiency. Aluminum comes in second at around 24%, followed by brass, galvanized iron, and stainless steel. A liner thickness of about 4 millimeters works well across all these materials. The key is high thermal conductivity: you want the absorbed heat to transfer quickly into the water rather than radiating back out.
The glass cover angle matters because it controls how well condensed droplets slide into the collection trough rather than dripping back into the basin. Research comparing tilt angles of 20, 31, 45, and 50 degrees found that angles between 20 and 31 degrees produced the highest daily yield. Steeper angles reduced output.
Water depth is counterintuitive. You might think a deeper basin would produce more water, but the opposite is true. A shallow layer of just 1 centimeter heats up faster and evaporates more efficiently than a deeper pool. Keeping the basin depth below 3 centimeters consistently outperforms deeper fills. In one controlled experiment, a still with a 20-degree tilt angle and 1 centimeter of water depth produced the highest daily output. The basin shape also plays a role: a width-to-length ratio of about 0.45 yields optimal results.
Where Solar Stills Are Used
Solar stills have a long practical history. During World War II, the military developed portable solar stills for life rafts, giving downed pilots and shipwrecked sailors a way to produce drinking water from the ocean. That survival application still exists today, with compact inflatable solar stills included in some marine emergency kits.
The primary modern use is in remote, arid regions where groundwater is brackish and conventional water treatment infrastructure doesn’t exist. Communities in places like the Tharparkar district of Pakistan, parts of sub-Saharan Africa, and remote Australian outback settlements use solar stills to convert saline groundwater into drinking water. Small solar desalination units designed for individual households can provide a continuous, low-cost supply of potable water in these settings.
Agriculture is another growing application. Solar stills paired with greenhouses serve a dual purpose: the still produces irrigation water from saline sources while the greenhouse structure itself can support crop growth. This combination is particularly useful in arid farming regions where freshwater for irrigation is scarce but sunlight is plentiful.
Building a Basic Solar Still
The simplest version is a single-slope basin still. You need an insulated, watertight box with a dark interior (painted black or lined with a heat-absorbing material), a sheet of glass or clear plastic angled across the top, and a trough along the lower edge of the glass to catch condensed water. The box sits in direct sunlight, you pour in the water you want to purify, and gravity does the rest as condensation runs down the glass into a clean container.
For a ground-level emergency still, the concept is even simpler. Dig a hole, place a container at the bottom, cover the hole with clear plastic sheeting, and set a small rock in the center of the sheet so it sags into a cone shape above the container. Moisture from the soil or from plant material placed in the hole evaporates, condenses on the plastic, and drips into the container. Output is low, often well under a liter per day, but it can be a lifesaving technique in a survival situation with no other water source.
Whether you’re building a permanent household unit or a temporary emergency version, the core physics remain the same: trap sunlight, heat water, collect the condensation. The gap between a crude survival still and a well-engineered one comes down to materials, geometry, and insulation, but the principle never changes.