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Solar Desalination Calculator

Size solar panels for off-grid reverse osmosis water desalination. Enter daily water need, salinity, and location — get solar array, battery bank for 24/7 operation, and cost per liter.

Water system specifications
liters/day
Solar desalination system sizing
1.6 kW solar → 1,000 L/day clean water
Daily m³ output1.00 m³/day
RO energy rate4.5 kWh/m³
Daily energy needed5.2 kWh/day
Solar array4 × 400W panels (1.6 kW)
RO system cost$800
Solar + battery cost$4,160
Total system cost$6,960
Cost per liter (all-in, 15yr)$0.0013/L
vs Municipal water ($0.003/L)Saves $624/yr vs municipal
vs Bottled water ($1.00/L)Saves $364,529/yr
Annual operating cost$49/yr
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How to Use This Calculator

Enter daily water production and feed water type

Start with how many liters of clean water you need per day. For reference: 50-100L/person/day covers basic domestic needs; 2-4L/person/day is drinking water only; agricultural use ranges from 1,000 to 50,000+ liters/day. Select your feed water salinity type — brackish water (wells, estuaries, aquifers) at 1,000-10,000 ppm TDS requires far less energy than seawater at 35,000 ppm TDS. The energy difference is critical for solar sizing.

Choose location and operation mode

Location determines peak sun hours, which directly sets the solar array size. Desert and tropical coastal locations (Middle East, sub-Saharan Africa, Pacific Islands) are ideal for solar desalination — high sun hours coincide with high water demand. For continuous 24/7 water production, check the battery toggle — this adds a battery bank sized to run the RO system through the night and on cloudy days.

Interpret the cost per liter

The cost per liter shown is the fully amortized cost over 15 years including capital repayment and ongoing membrane/maintenance costs. For comparison: municipal water in developed countries costs $0.001-0.005/L; bottled water costs $0.50-2.00/L. Solar desalination typically lands at $0.002-0.010/L — dramatically cheaper than bottled water and competitive with or better than trucked water in remote areas.

The Formula

Daily m³ = Daily Liters ÷ 1,000 Daily kWh (RO) = Daily m³ × RO Energy (kWh/m³) Daily kWh (Total) = Daily kWh (RO) × 1.15 (pumping/pre-treatment overhead) Solar kW (daytime) = Daily kWh × 1,000 ÷ PSH ÷ 0.82 efficiency Battery kWh (24/7) = Daily kWh × (24 - PSH) / 24 ÷ 0.80 DoD RO System Cost = Daily m³ × $800/m³/day capacity Solar Cost = Solar kW × $2,600/kW Total System Cost = RO Cost + Solar Cost + Battery Cost + $2,000 misc Annual Capex = Total Cost ÷ 15 years Annual Opex = Daily m³ × 365 × $0.02/m³ (membranes + maintenance) Cost per Liter = (Annual Capex + Annual Opex) ÷ Annual Liters

The 15% overhead factor covers pre-filtration (sediment and carbon cartridges), high-pressure feed pump, and post-treatment (remineralization or UV polishing). Modern seawater RO systems using energy recovery devices achieve 3.5-4.0 kWh/m³; older systems without energy recovery use 5-6 kWh/m³. The RO membrane replacement cost ($0.02/m³) covers typical replacement every 3-5 years at commercial membrane prices.

Example

Pacific Island — 1,000L/day community system

A Pacific island community of 20 people needs 1,000L/day of fresh water from seawater (35,000 ppm TDS). Local PSH is 5.2. They want daytime-only operation to minimize cost.

Daily water needed1,000 L/day (1 m³)
Feed waterSeawater (35,000 ppm)
RO energy4.5 kWh/m³
Location PSH5.2 PSH
OperationDaytime only

Result

Daily energy needed5.2 kWh/day
Solar array4 × 400W panels (1.6 kW)
Total system cost~$8,200
Cost per liter~$0.0015/L
vs Bottled waterSaves ~$360,000/yr

This 4-panel solar desalination system provides 50 liters per person per day of fresh water from seawater for the community, at a cost of $0.0015/liter — 667 times cheaper than bottled water. The system pays for itself versus trucked water in under 2 months.

FAQ

Energy consumption depends heavily on feed water salinity. Brackish water at 1,000-5,000 ppm TDS requires just 1.0-2.0 kWh per cubic meter (1,000 liters). Medium-salinity sources at 5,000-10,000 ppm need 2.5-4.0 kWh/m³. Seawater at 35,000 ppm requires 3.5-6.0 kWh/m³ depending on system design — modern SWRO systems with energy recovery devices achieve 3.5 kWh/m³, which is about 40% less than 10 years ago. Solar is ideal for RO because panels produce the most energy during peak demand periods.
The key difference is operating pressure and energy consumption. Seawater RO (SWRO) operates at 55-85 bar pressure due to seawater's high osmotic pressure (~27 bar for 35,000 ppm). Brackish water RO (BWRO) operates at 8-15 bar for water at 3,000-5,000 ppm. This means BWRO uses 3-4x less energy than SWRO and requires smaller, cheaper pumps. If you have access to brackish water (inland aquifers, wells near coast, estuaries), always prefer it over seawater — the economics are dramatically better. BWRO systems are also simpler to maintain and the membranes last longer.
For most community-scale systems, daytime-only operation with a water storage tank is more cost-effective than 24/7 operation with batteries. Store the daily water output in a large cistern or tank, which is far cheaper than the batteries needed for night operation. For example, a 10,000-liter tank costs $500-2,000 vs the $20,000+ in batteries needed for 24/7 operation of a comparable system. Run the RO system during peak sun hours (10am-4pm) and store the output. 24/7 operation makes more sense only when water storage is impractical (space constraints, hygiene concerns) or when water demand is truly continuous.
Solar desalination costs approximately $0.001-0.010 per liter depending on scale and salinity. For comparison: municipal water in the US costs $0.001-0.005/L; municipal water in developing countries varies widely ($0.002-0.02/L); trucked water in water-scarce regions costs $0.05-0.50/L; bottled water costs $0.50-2.00/L; emergency relief water (trucked or airlifted) can exceed $5.00/L. Solar desalination is roughly competitive with municipal water, dramatically cheaper than trucked or bottled water, and far cheaper than emergency supply. At scale (>10,000 m³/day), large solar desalination plants can reach $0.0005/L.
RO membranes typically last 3-7 years depending on feed water quality and maintenance practices. Seawater membranes tend to last 3-5 years; brackish water membranes 5-7 years with good pre-treatment. Key factors extending membrane life: (1) proper pre-filtration (5-micron sediment + carbon cartridges), (2) consistent operating pressure (avoid pressure spikes), (3) regular cleaning with approved chemicals, (4) not running dry. Biofouling in warm tropical waters can reduce life to 2-3 years without antiscalant dosing. Membrane replacement typically costs $100-500 per element depending on size and type.

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