Solar Still Water Production Calculator
Introduction
A solar still turns salty, dirty, or otherwise non-potable water into distilled water by using nothing more than sunlight, a warm basin, and a cooler condensing surface. In simple terms, the still copies the natural water cycle on a small scale. Liquid water absorbs solar energy, part of it evaporates, the vapor meets a cooler cover, and the condensed droplets run into a collection channel as cleaner water. That sounds elegantly simple, but practical planning is harder than the concept itself. People building a still for emergency use, off-grid living, science class demonstrations, or remote field work usually want the same answer first: how much water can this setup actually make in a day?
This calculator gives a quick estimate of daily distilled water production from three inputs that matter most in a first-pass design: basin area, local solar irradiance, and still efficiency. Basin area tells you how much sunlight-catching surface the still presents. Irradiance tells you how much solar energy arrives on that area over a day. Efficiency tells you what fraction of that incoming energy truly ends up driving evaporation and collection rather than disappearing through reflection, heat loss, imperfect condensation, or poor sealing. The result is not a laboratory-grade simulation, but it is very useful for comparing designs, sizing an emergency water plan, and deciding whether one still is enough or whether you need several running in parallel.
How to Use This Calculator
Start by entering the basin area in square meters. This is the active area where the feed water is spread and exposed to solar heating. If your still has a rectangular basin, multiply length by width. If it uses more than one tray or multiple equal basins, add those areas together. The number does not need to be perfect to the last decimal place, but it should reflect the real sun-exposed evaporation surface rather than the outside footprint of the whole device.
Next, enter solar irradiance in kilowatt-hours per square meter per day. That value describes the daily solar energy available at your location. Many weather and solar resource maps publish it as daily insolation. A cloudy temperate site may see closer to 3 or 4 kWh/m²/day, while a bright desert or tropical site may be around 6 or 7 kWh/m²/day. Use a daily average for your season of interest. If you are planning around the worst month rather than the best month, the calculator will give a more conservative and usually more realistic estimate for survival or resilience planning.
Finally, enter still efficiency as a percent. This is the most uncertain input, and that is normal. Homemade pit stills often fall in the low range because they lose heat easily and condense poorly. Better-designed basin stills with dark absorptive surfaces, good insulation, tight seals, and a well-angled cover can do much better. If you are unsure, try several runs: perhaps 20%, 30%, and 40%. That quick sensitivity check is often more helpful than pretending you know the exact efficiency in advance. Once you press Calculate, the result area shows estimated liters of distilled water per day. Use that number as a planning figure, then compare it against your real measured output after you build or test the still.
Formula
The energy logic behind the calculator is straightforward. Water does not evaporate for free; it requires latent heat of vaporization. At practical solar still temperatures, a useful planning value is about joules per kilogram, which is commonly expressed for this purpose as roughly 0.63 kilowatt-hours for each liter of water evaporated and collected. That means even a sunny day only produces modest volumes unless the still has enough area and a reasonably efficient design.
If a location receives kilowatt-hours per square meter per day, and your still has basin area , then the total solar energy available to the basin is . Not all of that energy becomes evaporation. Reflection, conductive losses to the frame, reradiation to the environment, and incomplete condensation reduce the useful share, so the calculator multiplies by an efficiency factor . The resulting daily water output follows this relationship:
Formula: m = (E A η) / L_v
where is the latent heat per liter used by this calculator. In plain language, the tool takes the solar energy arriving each day, keeps only the fraction your still can convert into useful evaporation, and then divides by the energy needed to make one liter of distilled water. That is why area and sunlight scale production upward, while low efficiency can sharply reduce the result even when the weather looks favorable.
The formula also explains why solar stills are physically elegant but volumetrically modest. People often expect a small still to make far more water than it can, simply because the sun feels intense. Yet evaporation is energy-hungry. A still with a small basin can work very well and still make only a few liters per day. This is not a flaw in the calculator; it is the reality that latent heat dominates the problem. For planning, that insight is useful because it encourages realistic sizing from the start.
Worked Example
Suppose you build a single-basin solar still with an active area of 1 square meter and place it in a sunny location that receives 5 kWh/m²/day. If you estimate your overall still efficiency at 30%, the expected daily production becomes liters per day. That is a helpful benchmark because it feels both tangible and realistic: enough for part of one person’s daily drinking needs, but not enough to support a family by itself.
This example also shows how the calculator should be interpreted. A result of about 2.4 liters per day is not a guarantee that every day will deliver exactly that amount. It is an estimate under average conditions. A clearer day may outperform it, while clouds, poor sealing, dirty glazing, or insufficient cooling of the condensing surface may underperform it. Still, the number is extremely useful for deciding whether to enlarge the basin, build a second unit, improve insulation, or adjust expectations about how much potable water you can reliably collect.
Interpreting the Result
Once you calculate liters per day, the most practical next question is whether that output matches your real water demand. Drinking, minimal cooking, and safety reserve needs can add up quickly, especially in hot climates. The result therefore works best as a planning number inside a larger water budget. If your estimate is 3 liters per day but your household needs 8 liters for basic consumption, the calculator is telling you immediately that you either need more still area, higher efficiency, supplemental purification methods, or stored reserve water for cloudy periods.
The table below gives a quick sense of scale at a fixed irradiance of 6 kWh/m²/day. It highlights a common lesson: production rises linearly with area, and meaningful but smaller gains come from better efficiency. In other words, a well-built small still is better than a poor one, but a larger area still matters enormously if you need more output.
| Area (m²) | Efficiency | Output (L/day) |
|---|---|---|
| 0.5 | 20% | 0.95 |
| 1.0 | 30% | 2.86 |
| 2.0 | 35% | 6.67 |
| 5.0 | 40% | 19.05 |
Reading the result well also means thinking about quality and maintenance. Distilled output is only useful if it is captured cleanly. A still that theoretically evaporates enough water can still disappoint if droplets fall back into the basin, mineral scale accumulates on the evaporating surface, or the collection channel leaks. The calculator quantifies expected production, but real success still depends on design details and routine upkeep.
Limitations and Assumptions
This calculator is intentionally simple, so it uses a steady energy-balance approach rather than a time-by-time thermal simulation. That means it does not model hourly swings in sunlight, ambient temperature, wind speed, basin depth, or night cooling in detail. In real operation, those factors matter. A windy day might help condensation by cooling the cover, but it might also steal heat from the basin. A shallow dark basin can warm quickly, while a deeper basin may respond more slowly. The estimate is best viewed as a practical average, not a minute-by-minute forecast.
Efficiency is the biggest source of uncertainty. Two stills with the same area in the same sunlight can perform very differently because of glazing angle, insulation, seal quality, absorptive coating, vapor path length, internal airflow, and how effectively condensed droplets are directed into the trough. If you are using the calculator before building anything, it is smart to test a range of plausible efficiencies rather than a single optimistic value. If you already own or built a still, compare actual measured daily output to the estimate and then adjust your assumed efficiency until the calculator matches reality more closely for future planning.
The water quality assumption also deserves care. Solar distillation is excellent at removing salts, many microbes, and suspended solids, but it is not a magic cure for every contaminant. Some volatile compounds can travel with the vapor. If the source water contains fuels, solvents, or other volatile chemicals, extra treatment steps may be needed. Likewise, concentrated residues left in the basin should be removed periodically. As salts and dirt build up, heat transfer and evaporation become less effective, which means the true efficiency drifts downward over time unless the system is cleaned.
Finally, the calculator reports output in liters per day under average input conditions, but real life includes downtime. Clouds happen. Covers fog. Collection channels clog. Users refill basins late or forget to flush brine. That is why emergency planners and off-grid households should not rely on a solar still as a single-point water solution unless they have generous redundancy and storage. The tool is strongest when used for comparison, sizing, and expectation setting. It tells you whether a concept is plausible, whether one design change matters more than another, and how many square meters of still area you may need before construction effort becomes worthwhile.
Planning and Sharing Output
After you run the calculation, treat the result as one line in a broader resilience plan. Write down the expected liters per day, the season you assumed, and the efficiency you used. That record makes later comparisons much easier. If your real-world still underperforms, you can inspect whether the basin is too reflective, the cover angle is poor, or the unit is losing heat around the edges. Small improvements often show up directly as a higher effective efficiency.
The estimate is also useful when coordinating with family members, community groups, or emergency teams. Sharing a realistic production figure helps everyone understand whether the still is a backup source, a teaching tool, or a meaningful contributor to daily drinking needs. A transparent number supports better decisions than a vague promise that the sun should provide enough water.
Mini-Game: Condensation Sprint
This optional canvas game turns the same idea behind the calculator into a fast visual challenge. Instead of plugging numbers into a formula, you manage the part of the still that often decides whether vapor becomes usable water: the cool condensation zone on the glazing. Clean vapor packets rise from the heated basin, and your job is to sweep the blue cooling strip along the glass so the vapor condenses and slides into the collection trough.
The game does not change the calculator result, but it reinforces the lesson that sunlight alone is not enough. A productive still also needs efficient capture. When you miss vapor or cool the wrong plume, potential output is lost. When you stay aligned with the strongest vapor stream, your collection efficiency rises, just as a better-built real still turns more of its solar input into actual liters of water.
Educational takeaway: higher irradiance creates more vapor, but output only becomes drinking water when the still condenses and captures that vapor efficiently.