# Tidal and ocean-current power: Efficiency

Perfectly predictable, yet idle for much of every tide

Source page: https://www.materialinsights.org/energy/e_tidal_power/efficiency/

What this tab covers How much of the energy available is actually captured or extracted, in per cent: theoretical limits, laboratory records, typical new equipment and in-service averages, each with exactly what is being measured.

Tidal stream turbines face the same Betz limit as wind turbines (about 59% of the kinetic energy in the flow) and current designs reach about 50%. Barrages use low-head hydro turbines, which are efficient, but they only generate for part of each tide.

Chart: Efficiency ranges
 Theoretical limit Commercial In service

#### Efficiency by measure

| Measure | Range | Type | What is measured |
|---|---|---|---|
| **Betz limit for an open-flow rotor** | 59%^[1](https://www.lms.ac.uk/sites/default/files/files/Mathematics/MPU/MathsInside_Turbine%20FINAL.pdf) | Theoretical limit | Maximum share of the flow's kinetic energy a turbine can extract, reached when the water is slowed to a third |
| **Tidal stream rotors, current designs** | 50%^[1](https://www.lms.ac.uk/sites/default/files/files/Mathematics/MPU/MathsInside_Turbine%20FINAL.pdf) | Commercial | Power coefficient: kinetic energy in the flow to shaft power |
| **Hydraulic (water) turbines** | 70 to 90%^[2](https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2014/Wave-Energy_V4_web.pdf) | Commercial | Water head to shaft power, as used in barrages |
| **La Rance barrage, capacity factor** | 25 to 40%^[3](https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2014/Tidal_Energy_V4_WEB.pdf)^[4](https://tethys.pnnl.gov/annex-iv-sites/la-rance-tidal-barrage) | In service | Average output over rated output; IRENA puts conventional barrages at about 25%, Tethys gives La Rance about 96 MW average from 240 MW (40%) |
| **MeyGen tidal stream array, 2024** | 19%^[8](https://www.ren21.net/gsr-2025/technologies/ocean-power/) | In service | Capacity factor from 10 GWh and four 1.5 MW turbines (calculated) |

#### Where the energy is lost

| Step | Loss |
|---|---|
| **Flow that must pass the rotor (Betz)** | at least 41% of kinetic energy^[1](https://www.lms.ac.uk/sites/default/files/files/Mathematics/MPU/MathsInside_Turbine%20FINAL.pdf) |
| **Barrage idle at slack water and while basins fill** | ebb-only plants generate about four hours a day^[3](https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2014/Tidal_Energy_V4_WEB.pdf) |
| **Downtime** | tidal arrays aimed to lift availability from about 70% to 90%^[3](https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2014/Tidal_Energy_V4_WEB.pdf) |

Tidal output is predictable years ahead, but it rises and falls with the tide and the spring-neap cycle, so capacity factors are modest. Ocean current turbines would face the same Betz limit but in a steadier flow; the US Department of Energy assumes a 30% device efficiency and a 70% capacity factor for them in resource estimates.

#### Sources

1. [Tidal Turbines (Maths Inside)](https://www.lms.ac.uk/sites/default/files/files/Mathematics/MPU/MathsInside_Turbine%20FINAL.pdf) London Mathematical Society
2. [Wave Energy: Technology Brief (turbine efficiencies, p. 11)](https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2014/Wave-Energy_V4_web.pdf) IRENA, 2014
3. [Tidal Energy: Technology Brief](https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2014/Tidal_Energy_V4_WEB.pdf) IRENA, 2014
4. [La Rance Tidal Barrage (Annex IV site page)](https://tethys.pnnl.gov/annex-iv-sites/la-rance-tidal-barrage) PNNL Tethys, 2024
5. [Renewables 2025 Global Status Report: Ocean power](https://www.ren21.net/gsr-2025/technologies/ocean-power/) REN21, 2025

## Other sections

- [Summary](https://www.materialinsights.org/energy/e_tidal_power/index.md)
- [Overview](https://www.materialinsights.org/energy/e_tidal_power/overview/index.md)
- [Correlation](https://www.materialinsights.org/energy/e_tidal_power/correlation/index.md)
- [Techniques](https://www.materialinsights.org/energy/e_tidal_power/techniques/index.md)
- [Resources and labour](https://www.materialinsights.org/energy/e_tidal_power/resources/index.md)
- [Impact on the biosphere](https://www.materialinsights.org/energy/e_tidal_power/biosphere/index.md)
- [Relationships](https://www.materialinsights.org/energy/e_tidal_power/links/index.md)