How does a hydroelectric power plant work? From water to electricity

IN DEPTH · POWER GENERATION

Approximately 7–8 minutes to read

Behind the dam, the water looks almost motionless. Below it, machinery can supply electricity to an entire town. Where does that energy come from, what exactly does the turbine do, and how can water act as an enormous battery?

When we say that a power plant makes electricity “from water”, it can sound like a chemical trick. Yet water molecules are not converted into electricity. The water passes through the equipment and continues downstream. The plant captures some of the energy associated with its position and movement. To understand the process, we need to follow two journeys: the route taken by the water and the transfer of its energy.

1. The energy is there before the turbine starts turning

Water in an elevated reservoir has gravitational potential energy. Here, “elevated” means that its surface is higher than the water downstream of the plant. The reservoir does not have to sit on a mountaintop. A dam or a weir across a river can create the necessary difference in level.

Imagine a book resting on a high shelf. If it falls, gravity sets it in motion. Water follows the same physical principle, except that a power plant directs its movement and extracts energy in a controlled way. Two things matter: how much water is available and how far it can descend.

This is why altitude above sea level is not the deciding factor. A reservoir a thousand metres above the sea does not automatically have more usable energy than a lowland dam. What matters is the available head: the difference between upstream and downstream water levels, with allowance for losses along the route to the turbine. ^[1]

2. Gravity, pressure and the journey to the turbine

Water enters a conduit, which may be a pressure pipe known as a penstock or a tunnel. Gates, valves and other controls determine how much water reaches the machinery. Gravity and the difference in hydraulic energy between the upper and lower water levels allow it to flow. In a closed pipe, energy is also associated with pressure; the water is not simply falling freely as it would over a waterfall.

At the turbine, the water acts on the blades of a rotating runner. Their shape directs the flow so that some of the water's energy is transferred to the machine. The turbine converts hydraulic energy into mechanical rotation. A shaft then carries that rotation to the generator. Turbine and generator are separate devices with different jobs, even though they work together as one system. ^[2]

Not all turbines look alike. A Kaplan turbine resembles a ship's propeller and is particularly suited to lower heads and large flows. A Francis turbine works across a broad range of conditions, often at medium to higher heads. A Pelton turbine uses water jets striking double-cup-shaped buckets and is suited to high heads and smaller flows. The machine is selected to match the location. ^[3]

3. How the generator turns rotation into electricity

Simply spinning a metal shaft is not enough. The generator relies on electromagnetic induction: a changing magnetic field can induce a voltage in a conductor. In a typical large hydroelectric generator, a rotor carrying electromagnets turns inside a stationary stator containing windings.

As the rotor moves, the magnetic field passing through the stator windings changes. This produces an alternating voltage, and current can flow when the generator is connected to an electrical circuit. The electromagnets are not an inexhaustible energy source. To keep producing electricity, the turbine must keep supplying mechanical power. Drawing electrical power from the generator creates a corresponding electromagnetic torque opposing its rotation.

A transformer then adjusts the voltage to a level suitable for delivery to the grid. For the same power transfer, a higher voltage allows a lower current and therefore reduces losses in the cables. Meanwhile, the water leaves through the tailrace and rejoins the river. The water continues downstream; the electrical energy enters the grid. ^{[2], [4]}

4. How much electricity can water produce?

Power output depends mainly on flow, head and efficiency. A simplified expression is P = ρ × g × Q × H × η, where ρ is water density, g is gravitational acceleration, Q is flow rate, H is usable head and η is conversion efficiency. An actual calculation must account consistently for hydraulic, mechanical and electrical losses.

Consider an illustrative example: 20 cubic metres of water per second pass through a turbine with a usable head of 50 metres. Assume an electricity conversion efficiency of 90%. Taking water density as approximately 1,000 kg per cubic metre gives an electrical output of about 8.8 MW. Operating continuously at that output for an hour would generate approximately 8.8 MWh of electricity.

This also explains the distinction between a megawatt and a megawatt-hour. The former measures power; the latter measures energy delivered over time. A power plant's capacity alone does not tell us how much electricity it will generate in a year.

What about a single cubic metre of water? Descending through 100 metres releases approximately 0.273 kWh of gravitational potential energy. At an assumed conversion efficiency of 90%, that gives roughly 0.245 kWh of electricity, before any further grid losses. In theory, that would run a 10-watt LED bulb for about 24.5 hours. These are our own illustrative calculations, not the specifications of a particular plant.

5. Not every hydroelectric plant needs an enormous dam

A reservoir hydropower plant can retain water for a period before releasing it. This gives its operator greater freedom to choose when to send water through the turbines. That freedom has limits: the reservoir may also provide water supplies, flood protection or other services.

A run-of-river plant relies mainly on the river's available flow and generally lacks a large water reserve for shifting generation over long periods. That does not necessarily mean it operates without any impoundment or control. River schemes can include weirs, canals and smaller reservoirs. ^[5]

Slovakia's Gabčíkovo illustrates the importance of a large flow. The main power station has eight Kaplan turbine-generator units rated at 90 MW each, giving a total of 720 MW. This figure refers to the main plant rather than the wider complex of smaller stations. Its scale shows how a powerful river can provide substantial energy without an exceptionally high mountain drop. ^[6]

6. Pumped storage: saving energy for later

A pumped-storage plant uses two reservoirs at different elevations. When storing electricity is useful, the facility consumes it to pump water from the lower reservoir to the upper one. Later, water flows back down through a turbine, returning some of that stored energy to the grid. ^[7]

This is not free energy. Pumping consumes more electricity than subsequent generation returns. As a simple example, if the entire cycle were 80% efficient, an input of 100 MWh would yield 80 MWh on discharge. The remainder would be lost. The value of the system lies in moving energy from a time when it is available to a time when it is needed.

The Czech plant Dlouhé stráně uses this principle. Its two reversible units have a combined generating capacity of 650 MW and operate with a head of approximately 511 metres. Much of the machinery is underground. The plant does not replace the energy source used for pumping; it provides a way to manage that energy more effectively. ^[8]

7. Why water matters in a grid with wind and solar

Electricity consumption changes continually. Output from wind and solar also varies. Many hydroelectric plants can adjust their output relatively quickly, helping to balance generation and demand. This flexibility has value in its own right, alongside the amount of electricity produced. ^[9]

Being able to respond does not mean being able to run indefinitely. Increasing flow through the turbines uses stored water more quickly. The operator therefore has to ask not only “how many megawatts can we deliver now?” but also “how long can we sustain that output?”. The same distinction applies to a battery: power capability and stored energy are different quantities.

8. Renewable does not mean impact-free

Generating hydropower does not require burning coal or gas. However, a dam can alter fish migration routes, water temperatures and sediment movement. Flooding land can affect ecosystems and communities. Decomposing organic material in reservoirs can also release greenhouse gases, with the scale of those emissions depending on local conditions. ^[10]

Water availability is another constraint. A prolonged drought can reduce both river flow and reservoir levels. A renewable resource is therefore not automatically a source of constant output. Rainfall and runoff from the catchment replenish the natural supply; an operator cannot simply order more water. ^[11]

At the beginning of this journey, we often find solar energy. It drives evaporation, allowing water to return to land as precipitation. Gravity then draws that water towards lower elevations. A hydroelectric plant captures some of the energy along the way and converts it into a form we can use at home. Not by turning water into something else, but by connecting flowing water, rotating machinery and a changing magnetic field.

The world is more fascinating when you understand it.

Sources

Sources checked on 24 October 2026. Numerical examples are our own illustrative calculations; the assumed efficiency is not a universal plant specification.