What tidal energy is, and why the tide is predictable

Tidal energy is electricity from the movement of water caused by the tides. Unlike almost every other renewable it is gravitational, not solar, which is why its timetable is known decades ahead.

Generation happens on the rush between the tides, not at the peaks. Output falls to nothing four times a day, but every one of those moments is known years ahead.

The energy is gravitational, not solar

Almost every other renewable is sunlight in disguise: wind is uneven solar heating, hydro is evaporation falling back down. The tide is not. It comes from the gravitational relationship between the earth, the moon and the sun.

The moon raises a bulge of water on the side of the earth facing it and another on the far side, and the earth rotates through both. That is why the tide keeps a timetable rather than a forecast.

Two highs and two lows, drifting daily

12h 25mbetween one high tide and the next

24h 50mfor the pattern to return, as the moon moves on

~2 weeksspring to neap, as range grows and shrinks

Most coasts see two highs and two lows in roughly 24 hours 50 minutes. Each high is about 12 hours 25 minutes after the last, so tidal generation drifts steadily later each day rather than repeating on a fixed clock.

The energy is in the current, not the height

The intuitive picture is water rising and falling, and that height difference is what a tidal barrage uses. But most modern development targets the horizontal rush of water between the high and the low. That current is where the kinetic energy is, and it peaks between the tides, not at them.

Why geography decides everything

Tidal range and current speed vary enormously by coast. A funnel shaped estuary or a narrow channel concentrates flow and can produce currents several times the open ocean average, which is why the resource sits in a handful of places rather than everywhere the way sunlight does.