Desalination Energy Cost Calculator
What this calculator estimates
Desalination can turn seawater or brackish water into usable freshwater, but it is energy-intensive. This calculator gives a planning-level estimate of:
- Specific energy consumption (SEC) in kWh/m³
- Total daily electricity use in kWh/day
- Daily electricity cost in $/day
The intent is to help you explore how volume, salt concentration, efficiency, and electricity price influence operating cost. It is not a detailed membrane/thermal plant design tool.
Inputs (with typical ranges)
- Water volume (m³/day)
- How much product water you want to produce per day. Small systems may be <100 m³/day; municipal plants may be tens of thousands of m³/day.
- Salt concentration (ppm)
- Total dissolved solids (TDS) as parts per million (mg/L). Rough guide: seawater ~35,000 ppm; brackish ~1,000–10,000 ppm. (Actual seawater varies by location.)
- System efficiency (0–1)
- A simplified factor representing how effectively your system converts input electricity into separation work (including the benefit of energy recovery). Higher means less electricity per m³. Typical modern reverse osmosis systems often behave as “moderate-to-high” efficiency compared with older equipment.
- Electricity cost ($/kWh)
- Your blended energy price (generation + delivery + demand charges averaged to $/kWh, if applicable). Many commercial rates fall roughly in the $0.08–$0.30/kWh range, but it can be lower or higher.
Methodology and formulas
Real desalination energy depends on feed salinity, temperature, recovery ratio, membrane performance, pressure losses, pretreatment, pumping, and (for thermal processes) heat integration. To keep the estimate simple, this calculator uses a linear salinity scaling anchored to a typical seawater reverse-osmosis SEC.
1) Specific energy consumption (kWh/m³)
We start with a baseline SEC at typical seawater salinity and then adjust for salinity and efficiency:
Where:
- SECref is a reference specific electricity use for seawater RO (commonly a few kWh/m³ for modern systems).
- S is salt concentration (ppm).
- Sref is reference salinity (ppm), typically ~35,000 ppm for seawater.
- η is the efficiency factor you enter (0–1). A higher η reduces the estimated SEC.
Interpretation: If salinity doubles, the estimate doubles; if efficiency increases from 0.5 to 0.75, SEC falls by one-third. This is a simplification but it matches the intuition that higher salinity and poorer efficiency increase power demand.
2) Total daily energy (kWh/day)
Once SEC is estimated, multiply by daily volume:
Energy (kWh/day) = SEC (kWh/m³) × Volume (m³/day)
3) Daily electricity cost ($/day)
Multiply daily kWh by your unit electricity price:
Cost ($/day) = Energy (kWh/day) × Price ($/kWh)
How to interpret the results
- SEC (kWh/m³) is best for comparing scenarios and technologies. It normalizes for plant size.
- Total kWh/day drives electrical infrastructure sizing (service capacity, generators, solar + storage sizing, etc.).
- $/day is a direct operating expense component. To estimate a water unit cost (e.g., $/m³), divide daily $ by daily m³ (and then add chemicals, labor, membranes, maintenance, brine disposal, financing, etc.).
Worked example
Scenario: You need 500 m³/day of product water from feed at 35,000 ppm. You assume η = 0.60 and electricity price is $0.12/kWh.
- Salinity ratio: S/Sref = 35,000/35,000 = 1.00
- SEC estimate: SEC = SECref × 1.00 × (1/η) = SECref/0.60
- Total energy: Energy = SEC × 500
- Cost: Cost = Energy × 0.12
If SECref were 3.5 kWh/m³ (a common order-of-magnitude for modern seawater RO electricity use), then SEC ≈ 5.83 kWh/m³, Energy ≈ 2,917 kWh/day, and Cost ≈ $350/day.
Note: Your actual SEC can be lower or higher depending on recovery ratio, energy recovery device performance, intake lift, and pretreatment needs. Use this as a directional estimate.
Scenario comparison (illustrative)
| Case | Volume (m³/day) | Salinity (ppm) | Efficiency (η) | Expected impact on SEC | Expected impact on $/day |
|---|---|---|---|---|---|
| Brackish water | 500 | 5,000 | 0.60 | Much lower than seawater (lower S) | Much lower (unless price is high) |
| Typical seawater | 500 | 35,000 | 0.60 | Baseline for comparison | Baseline for comparison |
| Higher salinity + poorer efficiency | 500 | 45,000 | 0.45 | Higher (higher S, lower η) | Significantly higher |
| Higher volume (scale in output) | 2,000 | 35,000 | 0.60 | Similar SEC | ~4× total $/day |
Limitations and assumptions (important)
- Linear salinity scaling: The model scales SEC linearly with ppm. Real systems can be nonlinear due to osmotic pressure relationships, recovery ratio, and pressure constraints.
- Efficiency is a lumped factor: η combines many effects (pump efficiency, membrane performance, energy recovery device, fouling). Two plants with the same η may still have different SEC.
- Does not model recovery ratio: Higher recovery generally increases required pressure and brine concentration; it can change SEC materially.
- Excludes intake/outfall and site hydraulics: Seawater intake lift, long pipelines, and brine discharge pumping can add significant kWh.
- Excludes pretreatment and post-treatment: Filtration, chemical dosing, remineralization, and cleaning energy are not separately modeled.
- Electricity only: Thermal desalination (MSF/MED) primarily consumes heat; converting that to an “electricity equivalent” depends on plant configuration and is not captured here.
- Not a guaranteed operating cost: Demand charges, time-of-use pricing, minimum bills, and generator fuel costs can make $/kWh vary through the day.
If you need engineering-grade results, use vendor performance curves, incorporate recovery ratio and feed temperature, and separate unit operations (intake pumping, high-pressure pump, pretreatment) into a proper energy balance.