Solar Water Purification Calculator

Compare UV sterilization, reverse osmosis, and solar distillation. Enter daily volume and location — get solar watts, battery Ah, system cost, and cost per liter for each method.

Water purification requirements

Daily water volume to treat?

liters/day

Source water type?

Treatment method?

Location (peak sun hours)?

Off-grid (adds battery storage for evening operation)

UV Sterilization (40W): Kills bacteria/viruses. Does not remove dissolved solids, chemicals, or heavy metals. Best for microbiologically safe source water.

Solar water purification system

400W solar — treats 100 L/day

Treatment energy40 mWh/L

Daily energy needed4 Wh/day

Solar panels needed1 × 400W (400W total)

Battery for evening use (48V)13 Ah (0.6 kWh)

Treatment system cost$800

Solar + battery cost$1,390

Total system cost$2,690

Cost per liter (10yr amortized)$0.0084/L

Annual savings vs purchased water ($0.50/L)$17,945/yr

Annual savings vs bottled water ($1.20/L)$43,495/yr

Method comparison at 100 L/day

UV Sterilization (40W)System: $1,840 | $0.0060/L

Reverse Osmosis / RO (500W)System: $10,750 | $0.0325/L

Solar Distillation (1500W)System: $62,305 | $0.1712/L

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How to Use This Calculator

Enter daily volume and source water type

Start with how many liters you need to treat per day. A family of 4 needs 20-30L/day for drinking water; 100-200L/day for all domestic needs. A community health clinic typically needs 200-1,000L/day. Source type (river, well, rainwater) helps identify which treatment method is appropriate — river water usually needs UV or RO; well water may need RO for dissolved minerals; rainwater is often cleanest and suitable for UV-only.

Select your treatment method

The three methods represent different trade-offs. UV sterilization (40W) kills all bacteria and viruses in seconds, is cheap and simple, but does nothing about dissolved chemicals, heavy metals, or salt. RO (500W) removes virtually everything — pathogens, heavy metals, pesticides, salt, fluoride — but wastes 2-3 liters of feed water per liter produced and requires a pressure pump and pre-filtration. Solar distillation (1,500W) produces the purest water possible but uses 20-30x more energy than UV per liter, making it suitable only for very small volumes or highly contaminated sources.

Off-grid vs grid-tied

For most deployments in water-stressed regions, off-grid is the default — no reliable grid exists. The battery option adds storage sized for ~12 hours of treatment system operation, covering evening hours when solar isn't available. If you're grid-tied, uncheck off-grid to see the simplified solar sizing without battery cost.

The Formula

Daily Energy (Wh) = Daily Liters × Wh per Liter by method UV: 0.04 Wh/L (40W system @ ~1,000 L/hr throughput) RO: 0.50 Wh/L (500W system @ ~1,000 L/hr throughput) Distillation: 850 Wh/L (1,500W still @ ~1.8 L/hr output) Solar Watts = max(Daily Wh / PSH / 0.82, Method Watts × 1.5) Panels = Solar Watts ÷ 400W (round up, minimum 1) Battery Ah (48V, off-grid) = Method Watts × 12 hrs ÷ (48V × 0.80) Treatment System Cost: UV $800, RO $3,500, Distillation $2,500 Solar Cost = Solar Watts × $2.50/W Battery Cost = kWh × $650/kWh Total = Treatment + Solar + Battery + Misc Cost per Liter = (Total ÷ 10yr + Annual Opex) ÷ Annual Liters

The solar array is sized to at minimum supply the treatment device's rated wattage (multiplied by 1.5 safety factor), even if the daily energy calculation would suggest fewer panels. This ensures the treatment system always gets adequate power on an average day. For UV, the minimum is typically 1 panel even for small volumes — the UV lamp needs its rated power to be effective, regardless of how few liters you treat.

Example

Rural health clinic — 500L/day RO system in South Asia

A rural health clinic in India needs 500 liters/day of clean water from river source for patient care, sterilization, and staff. They want an off-grid RO system for best water quality. Local PSH is 5.5.

Daily volume500 L/day

SourceRiver water

MethodReverse Osmosis (500W)

LocationSouth Asia (5.5 PSH)

ModeOff-grid

Result

Daily energy0.25 kWh/day

Solar panels2 × 400W (800W)

Battery (48V)~16 Ah (0.8 kWh)

Total system cost~$6,800

Cost per liter~$0.0037/L

vs purchased waterSaves ~$89,000/yr

For the clinic, a 2-panel RO system provides 500L/day of clean water at $0.004/liter — compared to $0.50/liter for purchased clean water or $1.20 for bottled water. The system pays back against purchased water costs in under 2 months, and the clinic has reliable clean water for patient care regardless of local water supply reliability.

FAQ

Which is better: UV, RO, or solar distillation for purifying water? ▼

The best method depends on your contamination profile: UV is ideal when your only concern is biological contamination (bacteria, viruses, protozoa). It's cheap, fast, and uses very little energy — but it does nothing for heavy metals, pesticides, or dissolved salts. If your water is clear and chemically safe, UV is usually the right choice. RO is the most versatile — it removes pathogens, dissolved solids, heavy metals, fluoride, arsenic, and chemicals. It wastes 2-3 liters of feed water per liter produced and needs a pressure pump, but it handles the widest range of contaminants. For river water or wells with unknown chemistry, RO is the safest choice. Distillation produces the purest water and can handle heavily contaminated or saline sources, but at 850+ Wh/liter, it's 20,000x more energy-intensive than UV — practical only for very small volumes.

Does UV purification make water safe to drink? ▼

UV purification kills or inactivates virtually all pathogens — bacteria (E. coli, Salmonella), viruses (including hepatitis A, norovirus), and protozoa (Giardia, Cryptosporidium) at appropriate UV doses. However, UV has critical limitations: (1) It requires relatively clear water — high turbidity blocks UV light and protects pathogens. Pre-filter to below 1 NTU for UV to work effectively. (2) UV does not remove chemical contaminants — arsenic, lead, nitrates, pesticides, fluoride remain after UV treatment. (3) UV leaves no residual disinfection — water can be recontaminated after treatment if storage containers aren't clean. For biologically contaminated clear water from a clean well or protected spring, UV alone is excellent. For river water or unknown sources, use RO or UV + carbon filter combination.

How much water does an RO system waste? ▼

Traditional small RO systems waste 50-75% of input water as "concentrate" or "brine reject." For every liter of clean water produced, 2-3 liters go to drain. Modern systems with recirculation or energy recovery can reduce this to 20-30% waste. This matters significantly for water-scarce applications — if you need 500L/day of clean water, plan for 1,500-2,000L/day of feed water input. The concentrate isn't necessarily toxic — for low-TDS wells, it can often be used for irrigation. For applications where water efficiency is critical, undersink "zero waste" RO systems with permeate pumps achieve 1:1 ratios, though at higher cost and slightly lower flow rates.

How many solar panels does a UV water purifier need? ▼

A UV water purification system typically needs just 1 solar panel for most home or small community applications. A 40W UV lamp treating 1,000 liters per hour uses only 0.04 Wh per liter — treating 200 liters per day requires just 8 Wh of energy, which even a tiny 50W panel generates in minutes. The practical minimum is 1 × 400W panel to ensure reliable operation throughout the day, including startup surges. For a small family treating 50-100L/day, a 20-40W panel with a 10Ah battery is technically sufficient, but a single 400W panel provides abundant margin and can also charge phones, power lights, or run other small appliances.

Can I use rainwater with UV purification? ▼

Rainwater is generally the cleanest source water for UV purification. Collected from a clean roof into a covered tank, rainwater is typically low-turbidity and low-TDS, making UV treatment highly effective. Key precautions: (1) First-flush diverters — discard the first 1-2mm of rain per rainfall event, which carries dust, bird droppings, and debris. (2) Fine mesh screen on tank inlets to prevent insects and debris. (3) Covered, sealed storage tank to prevent recontamination. (4) UV system immediately before point of use, not at tank level. In urban or industrial areas, rainwater may contain dissolved pollutants (acid rain, heavy metals) — test before assuming UV alone is sufficient. For those contaminants, add a carbon filter before the UV lamp.

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