Tidal and ocean-current power: Efficiency

Perfectly predictable, yet idle for much of every tide

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	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	Commercial	Power coefficient: kinetic energy in the flow to shaft power
Hydraulic (water) turbines	70 to 90%²	Commercial	Water head to shaft power, as used in barrages
La Rance barrage, capacity factor	25 to 40%³⁴	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	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 energy¹
Barrage idle at slack water and while basins fill	ebb-only plants generate about four hours a day³
Downtime	tidal arrays aimed to lift availability from about 70% to 90%³

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

Tidal Turbines (Maths Inside) London Mathematical Society
Wave Energy: Technology Brief (turbine efficiencies, p. 11) IRENA, 2014
Tidal Energy: Technology Brief IRENA, 2014
La Rance Tidal Barrage (Annex IV site page) PNNL Tethys, 2024
Renewables 2025 Global Status Report: Ocean power REN21, 2025