How do different types of renewable energy work? Sun, wind and water

Solar generation in the EU grew by more than 20 % in 2025 and reached 13 % of all electricity, according to Ember’s European Electricity Review 2026. Wind supplied 17 %. Together they produced more power than coal, gas and oil combined for the first time, while hydropower, the oldest renewable of the three, had a dry year and fell by 12 %.

Those three sources make electricity in three physically different ways. One has no moving parts at all, one turns a rotor in the open air, and one lets water fall through a machine that has barely changed in principle for a century. The difference is not only of interest to engineers. It decides how the grid around them has to be designed.

Solar: light straight into current

A solar cell is a thin wafer of silicon treated so that one layer has a surplus of electrons and the other a shortage. Where the two layers meet, an electric field forms. When light strikes the cell, its energy knocks electrons loose, and the field pushes them in one direction. Connect the two faces of the cell with a wire and a current flows. Nothing moves except charge.

What comes out is direct current, and its size follows the light. It rises through the morning, peaks around midday, drops under cloud and stops at dusk. To reach a European socket it passes through an inverter, which turns it into alternating current at 50 Hz and holds it in step with the grid.

The inverter is the part that matters for the system. It decides how the plant behaves when the grid voltage dips or the frequency moves, and grid operators write their connection rules around it.

Wind: moving air, turning mass

A wind turbine takes kinetic energy from the air. The blades are shaped like aircraft wings; the wind flowing over them creates lift, which turns the rotor, which drives a generator. The energy in the wind rises with the cube of its speed, so a doubling of wind speed gives eight times the power. That single fact explains why developers spend years measuring a site before they build, and why the North Sea, with strong and steady winds, draws so much investment.

No turbine can take all of that energy. If it stopped the air completely, no new air could flow through. The theoretical ceiling, the Betz limit, is about 59 % of the kinetic energy passing through the rotor, and real machines stay below it.

Most modern turbines connect to the grid through power electronics, much like solar inverters. The rotor turns at whatever speed suits the wind, and a converter matches the output to the grid frequency. That makes the turbine efficient, but it also decouples its spinning mass from the grid.

Offshore wind adds distance. A wind farm far out at sea usually sends its power to shore over a high-voltage direct current cable, because alternating current over long subsea cables loses too much of its capacity to charging the cable itself. The site’s HVDC guide explains why, and the demonstration on HVDC links for offshore wind shows the kind of converter problems the European research programme BEST PATHS worked on between 2014 and 2018.

Hydro: falling water, stored power

A hydroelectric plant turns the energy of falling water into electricity. Water held behind a dam flows down a pipe, called a penstock, and spins a turbine connected to a generator. The power available depends on two things: how far the water falls and how much of it flows. A tall dam with a modest river can match a low weir on a very large one.

Hydro has two properties that sun and wind lack. It can store energy, because a full reservoir is simply energy waiting to be released. And its generators are large rotating machines locked to the grid frequency, just like those of coal, gas and nuclear plants.

Pumped storage goes a step further: a plant with two reservoirs pumps water uphill when power is cheap and releases it when power is short. Norway, which generates nearly all its electricity from hydropower, plays this balancing role for its neighbours through long cables such as NordLink to Germany and NorNed to the Netherlands. When German wind output is high, Norway can import power and hold its own water back.

What the grid needs from them

Here the three technologies split into two groups. Hydro, like a conventional power station, connects through a synchronous generator: a heavy rotor that turns in lockstep with the grid at 50 Hz. If a large plant suddenly trips, all of those spinning machines slow down together and release some of their stored rotational energy. That inertia slows the fall in frequency and gives the automatic reserves a few seconds to respond.

Solar panels and most wind turbines connect through power electronics and provide no inertia on their own. As their share rises, the grid has less spinning mass to lean on. Operators can respond in several ways: keep some synchronous machines running, add synchronous condensers (rotating machines that produce no power but add inertia and voltage support), or require new converters to be grid-forming, meaning they set their own voltage and frequency reference and behave more like a generator than a follower.

You can see why this matters in the account of the Iberian blackout of April 2025, where voltage control, not a lack of generation, sat at the centre of the investigation. The glossary has short definitions of inertia, grid-forming control and the other terms that come up there.

Location is the other constraint. The best wind is at sea and in the north, the best sun is in the south, and the largest reservoirs are in the Alps and Scandinavia, while demand sits in cities and industrial regions far from all three. Moving that power is the job of transmission lines and interconnectors, and it is where much of the effort of the next decade will go. Ember counted 14 EU countries in which wind and solar already out-generated fossil fuels in 2025; how many of them could keep the lights on through a still, dark week in January without their neighbours is a different question.