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Hydroelectric power in Britain

Hydropower, which harnesses energy from moving water, is a small but important source of low-carbon, renewable electricity generation and storage in the UK.

This guide explains what hydropower is, how the different types of schemes work, where Britain’s largest schemes are, and how businesses can use hydropower with or without generating it themselves.

Contents:

• How does hydroelectric power work?
• What are the main types of hydropower?
• What are hydroelectric dams?
• Is hydroelectric power renewable?
• Why doesn’t the UK use more hydropower?

What is hydroelectric power?

Hydropower is electricity generated from the flow or fall of water to spin a turbine connected to a generator.

It is the world’s largest source of renewable electricity, supplying roughly 14% of all the power generated globally, though rapidly growing solar and wind energy are now closing the gap.

Its appeal comes from running on the natural water cycle rather than burning fuel, making it a renewable, low carbon and generally low-cost source of electricity.

How does hydroelectric power work?

Every hydroelectric scheme runs on the same principle, turning the energy of moving water into electricity. The details vary by design, but the flow below covers each stage.

1. Water flow is prepared for power generation

Hydro schemes work by harnessing the energy of water moving downhill. How that water is prepared depends on the type of scheme:

• Dam-and-reservoir: The classic setup that holds a large volume of water behind a dam to release on demand.
• Run-of-river: Diverts part of a river’s natural flow so power output rises and falls with the river.
• Pumped storage: Pumps water up to a reservoir at height with spare electricity when demand is low, then releases it back down to generate when demand is high.

This gives the water the vertical height, known as the head, which together with the flow rate determines how much energy is available. The greater the head and the flow, the more power can be generated.

2. Water is directed to the turbine

The energy of the water is now converted into movement by letting it flow downhill to the turbine, though how this happens depends on the scheme:

• Dam and pumped schemes: These release water on demand, sending it down through a large pipe called a penstock. A high-head mountain scheme may drop the water hundreds of metres, so it reaches the turbine with considerable force.
• Run-of-river schemes: Direct the river’s natural flow through the system more or less continuously, relying more on the volume of water than on a steep drop.

3. The moving water spins a turbine

The water strikes a turbine and spins it, turning its energy into mechanical motion. The turbine design is matched to the scheme:

• Pelton turbines: For high-head, low-flow sites, using narrow jets to strike cups around a wheel.
• Francis turbines: The most common, handling the medium heads and flows of many large dams.
• Kaplan turbines: A propeller-like design with adjustable blades, built for low-head, high-flow schemes such as run-of-river.

4. The turbine drives a generator that generates power

The turbine shares a shaft with a generator, so spinning one spins the other.

• Generates alternating current (AC): The rotation at the generator sweeps electromagnets past coils of wire, pushing electrons along the wire to create an electric current. Because the shaft spins in a continuous cycle, the output is alternating current (AC), which is what the grid runs on.
• Power generation is controlled: Operators adjust power output by controlling how much water reaches the turbine and spins it. The faster it spins, the more electricity is generated.

5. Electricity is transmitted to consumers

Freshly generated electricity is at too low a voltage to travel far, so a step-up transformer first raises it to a very high voltage. From there, its path depends on the scheme:

• Grid power: Most hydro schemes feed the national transmission network that carries power long distances, running at 275kV or 400kV in the UK. Substations then step it down onto the distribution network at levels such as 33kV and 11kV, and finally to the 230V that reaches homes and businesses.
• Private power: Smaller generators often supply a single site such as a farm, estate or factory directly with off-grid energy. This can cut a business’s energy bills and its reliance on the grid, and is common for the modest output of many run-of-river installations.

What are the main types of hydropower?

There are various types of hydropower to suit various geographic contexts and power requirements.

• Storage (dam-and-reservoir) hydro: Water is held behind a dam and released through turbines on demand. This is the classic large-scale form, giving operators tight control over when power is generated.
• Run-of-river hydro: Uses a river’s natural flow with little or no storage, so output follows the river and dips in dry spells. It needs no large reservoir, making it cheaper and less disruptive, but less controllable.
• Pumped storage hydro: Two reservoirs used as a giant battery, pumping water uphill with cheap surplus power and releasing it at peak demand. It stores energy rather than being a net generator.
• Micro and small-scale hydro: Small installations, often on a single watercourse, powering a farm, estate, business or community. Individually modest, but well suited to the UK’s many smaller rivers and old mill sites.

In the UK, storage and run-of-river schemes dominate the Scottish Highlands, pumped storage sits in a handful of large plants in Scotland and Wales, and micro-hydro is scattered across rural sites nationwide.

What are hydroelectric dams?

A hydroelectric dam is a barrier built across a river to hold water back, raising it to a height where its stored energy can be used to generate electricity, and combines several parts:

• Reservoir: The body of water held behind the dam, acting as a store of energy that can be drawn on when needed.
• Head: The vertical distance the water falls from the reservoir to the turbine. The greater the head, the more energy each litre carries.
• Intake and penstock: The controlled opening and large pipe that channels water down to the turbines.
• Turbines and generators: Housed at the base, where the falling water is converted into electricity.
• Spillway: A safety channel that lets excess water bypass the turbines during heavy rainfall, protecting the dam from overtopping.

Is hydroelectric power renewable?

Hydropower is renewable as it runs on the natural water cycle, replenished endlessly by rainfall and gravity, so unlike coal, oil or gas, its fuel never runs out.

However, renewable does not mean low impact, as schemes still have environmental and social consequences.

Large reservoirs can flood valleys and displace the communities and farmland that once occupied them, while dams block fish migration and alter the river flow downstream, which is why new UK projects face close environmental scrutiny.

Why doesn’t the UK use more hydropower?

The UK has plenty of rain and, in Scotland and Wales, plenty of hills, yet hydropower supplies only around 2% of its electricity mix. The reasons come down to the scale of the resource and, above all, the competition:

• Cheaper, easier rivals: Wind and solar have fallen dramatically in cost and can be built almost anywhere. Offshore wind is the clearest example, since it sits out at sea and largely avoids the local objections, land access disputes and river-ecology concerns that slow hydro down. Given the choice, investment flows to these instead.
• The resource is modest: Compared with truly mountainous countries like Norway, which draws about 95% of its power from hydro, the UK’s terrain is low-lying with small catchments. Even the Scottish Highlands offer relatively little head and flow, capping how much hydro is physically worthwhile.
• The big sites are already built: Most of the UK’s viable large-scale hydro was developed in the mid-20th century, leaving little large capacity to add.
• What’s left is small and contested: The remaining potential is mostly many sub-5MW schemes, which struggle to justify their cost and planning effort, especially with cheaper renewables available and local opposition to new dams and diversions.

Can businesses use hydropower?

Yes, either through self-generation if the site has running water, or through a green electricity contract.

Self-generation through a micro hydropower generator

Businesses with access to flowing water, such as farms, rural estates, mills, distilleries and some industrial sites, can install small or micro-hydro schemes to generate electricity on-site for their own consumption.

Old watermill sites are often ideal, since the necessary drop and water rights may already exist. The catch is that hydropower is highly site-specific and tightly regulated. Before generating, a business will typically need:

• Planning permission: Almost always required, since schemes involve built structures such as an intake, turbine house and sometimes a weir.
• A water abstraction or impoundment licence: Needed to take or hold back water. This comes from the Environment Agency in England, Natural Resources Wales (NRW) in Wales, and the Scottish Environment Protection Agency (SEPA) in Scotland under its own Controlled Activities Regulations.
• Environmental sign-off: Regulators will only licence a scheme if it protects wildlife, for instance through a fish pass or a minimum required flow.

Because these run in sequence and mistakes are costly, most businesses start with pre-application advice from the relevant regulator and a specialist consultant.

Buying hydropower through a business energy contract

It is not possible to buy electricity 100% derived from hydro because, in practice, power from all sources is mixed in the grid and it’s impossible to discern. However, it is possible to buy a contractual claim to renewables, including hydro, tracked through certificates.

The main routes are:

• A green tariff: The most practical alternative. Suppliers back “100% renewable” tariffs with Renewable Energy Guarantees of Origin (REGOs), one issued per megawatt hour of certified renewable generation. These tariffs usually blend wind, solar and hydro rather than isolating one source, though some business energy suppliers can offer hydro-weighted options.
• A corporate power purchase agreement (PPA): A direct contract with a named generator, such as a specific hydro scheme, giving a much closer link between your consumption and the source. This suits larger businesses and typically runs for several years.

Hydroelectric power stations in Britain

Britain’s largest hydroelectric stations are clustered in the Scottish Highlands, where the terrain gives them the head and rainfall they need.

Their output is highly predictable, which lets them provide reliable baseload power to the Scottish distribution network rather than on-demand bursts.

The largest ones are owned and operated by business energy suppliers, and the electricity they generate is certified under the REGO scheme and used to back green energy tariffs.

Here are the five largest by capacity:

Pumped-storage hydroelectric plants in Britain

Pumped storage hydro (PSH) is the main source of large-scale energy storage on the national grid, and Britain’s fleet is concentrated in the mountains of Wales and Scotland.

With a filled reservoir, these can switch from standby to full power generation within seconds. These are the five largest sites in operation:

Future pumped-storage hydro: Coire Glas PSH

Due to the grid’s growing need to balance intermittent wind and solar, SSE is going ahead with Coire Glas on Loch Lochy in the Highlands.

This proposed scheme of up to 1.5 GW will more than double Britain’s pumped storage capacity and be the first large plant of its kind built in over 40 years.

It was long stalled by the lack of a funding model; it moved a step closer in 2026 when Ofgem shortlisted it for new long-duration storage support, alongside a wider pipeline of similar projects.

British water companies generating hydro power

Hydropower has also been taken up by the British water industry, which is well placed for it, since it already owns reservoirs and moves large volumes of water around the country under pressure.

Scottish Water is the standout example. It has been installing turbines since 2010 and now runs hydro at more than 28 of its sites, totalling around 5.5 MW of capacity, with plans to add more.

Its scheme at Firrhill in Edinburgh is the first UK hydro plant hosted within a city, generating electricity from treated drinking water as it passes through the network.

United Utilities, the water infrastructure provider for North West England, takes a similar approach at its Oswestry plant. The electricity generated on site helps power a sewage treatment works, with any surplus exported to the local grid.

The appeal is twofold. Water in Britain carries a high carbon footprint, so on-site hydro is an effective way to cut both the carbon intensity and the wider environmental impact of the water industry, while also offsetting the energy costs of running treatment works.

At AquaSwitch, we help businesses get more from their water supply, including reducing their environmental impact. We help companies compare business water suppliers, including those offering green tariffs. Compare the market and switch business water supplier today.

Hydropower FAQs

Our business water and energy experts answer commonly asked questions regarding UK hydropower:

How efficient are hydroelectric power stations compared with other renewables?

Very efficient. A modern hydro turbine converts around 85 to 90% of the energy in moving water into electricity, among the highest of any generation technology.

Direct comparison with wind and solar is not possible since those are usually measured by capacity factor or panel efficiency rather than conversion efficiency.

What sets hydro apart is its controllability, since a storage scheme can generate on demand, whereas wind and solar only produce when the weather allows.

Can hydroelectric power stations generate electricity during droughts?

It depends on the type of scheme. A storage scheme with a large reservoir can keep generating through short dry spells by drawing on its stored water, though a prolonged drought will lower reservoir levels and eventually force reduced output.

Run-of-river schemes are the most exposed, since their output follows the river and can fall sharply or stop altogether in a severe drought. Pumped storage is largely unaffected, as it recycles the same body of water rather than relying on fresh inflow.

Do hydroelectric dams help reduce flooding?

Sometimes, but it is not their main purpose. A reservoir can be managed to hold back water and soften a flood peak downstream, which is a genuine benefit of some multi-purpose schemes.

However, a reservoir kept full for generation has little spare room to absorb a surge, so the flood benefit depends heavily on how the reservoir is managed.

Dedicated flood-control reservoirs, which are deliberately kept part-empty, do this job far better than a hydro dam optimised for power.

How long can a hydroelectric power station operate before major refurbishment is needed?

Longer than almost any other form of generation. The civil structures such as dams and tunnels can last a century or more, while turbines and generators typically run for 30 to 50 years before a major refurbishment.

This longevity is one of hydro’s great strengths. Many of Britain’s stations were built in the 1930s to 1950s and are still in service today, often with upgraded equipment, which spreads their high upfront cost across an exceptionally long life.

Can abandoned mills be converted into small hydroelectric schemes?

Yes, and they are among the most practical sites for micro-hydro. Britain has thousands of former watermill sites that already have a weir, a drop and, in some cases, historic water rights, so much of the groundwork exists.

A conversion still needs modern approvals, though, including planning permission, an abstraction licence and measures to protect fish, so a historic right to use the water does not remove the need to satisfy today’s regulators.

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