Tides: Fluctuation in time
Perfectly predictable, but always shifting against demand
What this tab covers How availability changes over seconds, days, seasons, years and longer, how predictable it is, and what that means for the plants that use it.Tides vary strongly within a day and a fortnight but are almost perfectly predictable years ahead, because they follow the motions of the Moon and Sun rather than the weather.
Variation by time scale
| Time scale | What happens | How large |
|---|---|---|
| Hours (semi-diurnal cycle) | Two high and two low tides per lunar day; output falls to zero at slack water when the current turns7 | High tides 12 hours 25 minutes apart; lunar day 24 hours 50 minutes |
| Fortnightly (spring-neap cycle) | Spring tides at new and full moon when Sun and Moon pull together; neap tides a week later when they pull at right angles8 | Two springs and two neaps per lunar month; stream power scales with the cube of current speed, so neaps yield far less |
| Daily shift | Peaks arrive about 50 minutes later each day, so they drift in and out of phase with demand peaks7 | About 50 minutes per day |
| Weather and seasons | Wind and air pressure push water levels above or below the predicted tide (low pressure raises it, high pressure lowers it), but do not change the underlying cycle12 | Departures from prediction during storms; the astronomical cycle itself is unaffected |
How predictable it is
Excellent: tides can be calculated decades ahead from astronomy. The variability is large but known, which makes it easier to plan around than wind or sun.
Capacity factors and utilisation
| Plant or measure | Value |
|---|---|
| Conventional tidal barrage (load factor) | about 25%2 |
| La Rance (calculated from IRENA's 540 GWh/yr and 240 MW; Tethys's 600 GWh/yr would give 29%) | about 26%2 |
| MeyGen tidal stream array, 2024 (calculated from 10 GWh and 6 MW) | about 19%6 |
| Developer target for tidal stream arrays | 25% rising to 40%2 |
A tidal plant runs to a strict rhythm set by the Moon. A barrage generates in bursts on the ebb (and sometimes the flood), with gaps at slack water; a stream turbine follows the current speed, peaking about four times a day. Over a fortnight the output swells and shrinks with the spring-neap cycle.
Because the schedule is known in advance, grid operators can plan for it, but the peaks drift by about 50 minutes a day relative to human demand, so tidal power still needs storage or other generation to fill the gaps.
Sources
- Tidal Energy: Technology Brief IRENA, 2014
- Renewables 2025 Global Status Report: Ocean power REN21, 2025
- Tides and Water Levels: The lunar day NOAA National Ocean Service, 2024
- Tides and Water Levels: Variations (spring and neap tides) NOAA National Ocean Service, 2024
- Tides and Water Levels: What affects tides in addition to the Sun and Moon? NOAA National Ocean Service, 2024