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Fluctuating generation

fluctuating generation wind energy photovoltaics renewable energies power system forecasts flexibility storage grid expansion

Fluctuating generation refers to electricity production whose available output varies over time because it depends on the natural supply of energy. In the power system, this mainly means wind power and photovoltaics plants. Their feed-in follows wind speed, cloud cover, the position of the sun, the season and the weather situation. It can be technically limited or curtailed. Without an additional energy source, however, it cannot be increased at will when there is little wind or no sun.

The term therefore does not mean a lack of controllability in the narrow technical sense. Modern wind and solar plants can reduce their feed-in very quickly, provide reactive power, comply with grid connection rules and, in some cases, be operated in a grid-supportive way. What fluctuates is above all the available primary energy supply. A photovoltaic plant can deliver a lot of power at midday when the sun is shining and none at all at night. A wind turbine reaches its rated power in strong wind and feeds in nothing in a calm. In very strong storms it shuts down to protect itself. What matters first is the electrical power in kilowatts, megawatts or gigawatts. Over time, this results in an amount of energy in kilowatt-hours, megawatt-hours or terawatt-hours.

Power, energy and availability

With fluctuating generation, power and energy are often confused. The installed capacity of a plant indicates the maximum electrical power it can provide under suitable conditions. It does not say how much electricity the plant actually produces in a given period. A photovoltaic plant with ten kilowatts of installed capacity delivers those ten kilowatts only if irradiation, module temperature, orientation and inverter operation are right. Its annual output depends on location and weather. An analogy: installed capacity is the diameter of a pipe. How much water actually flows through is decided by the weather.

This distinction is central to assessing the power system. A power plant fleet can generate very large amounts of electricity from wind and sun over the year and still provide little power in individual hours. Conversely, there are hours with very high feed-in in which more wind and solar power is available than consumption and export possibilities can absorb. This difference gives rise to requirements for flexibility, storage, grids and controllable generation.

The term availability is also used differently here. For conventional power plants, it often means whether the plant is technically ready for operation. A wind or photovoltaic plant can be technically available even though wind or sun is currently supplying too little energy. For security of supply, therefore, firm capacity counts. This is the share of capacity that can be reliably counted on in critical situations. For fluctuating generation, it lies well below installed capacity. How high it is depends on statistical assumptions, the spatial distribution of the plants and the supply situation under consideration, such as a low-wind winter evening.

Distinction from renewable, decentralised and volatile

Fluctuating generation is not the same as renewable generation. Biomass and biogas plants use renewable energy carriers that can be stored. Depending on design and fuel logistics, they can therefore be operated in a plannable way. Storage hydropower plants can shift energy over time. Run-of-river hydropower plants also depend on the available supply, namely the river's flow. However, their output usually changes slowly and mainly seasonally. In common usage, they are therefore usually not counted as fluctuating generation in the narrower sense. With wind and photovoltaics, short-term changing weather determines when and how much energy is available.

Equating it with decentralised generation is also imprecise. Photovoltaic plants on house roofs often feed into the low-voltage grid. Large solar parks and offshore wind farms, by contrast, are connected to higher grid levels. Conversely, decentralised generation can be controllable, such as a combined heat and power unit or a biogas plant. The effect on the system therefore depends not only on location but also on connection capacity, grid level, forecast quality, controllability and market integration.

Volatile is often used as a synonym but is less precise. Volatility generally describes strong fluctuations, including in prices or demand. Fluctuating generation names the cause more precisely: the natural energy supply changes. This makes it possible to separate technical questions from market phenomena. Electricity prices can fluctuate because demand, fuel prices, grid congestion, power plant outages and fluctuating feed-in interact. Fluctuating generation is one influencing factor here, but not the whole explanation.

Why forecasts become an operating resource

Wind and solar power do not flow into the grid at random. With weather models, measurement data and short-term forecasts, feed-in can be predicted quite accurately, especially for large plant fleets and short periods. Forecast errors nevertheless remain important, because generation and consumption in the grid must be in balance at every moment. Even small deviations between expected and actual feed-in can make balancing energy, control reserve or changed schedules of other plants necessary.

Fluctuating generation thus changes grid and market operation. Balance responsible parties notify expected feed-in and consumption. Transmission system operators maintain the system balance and procure control reserve. Distribution system operators must deal with local feed-in peaks when many photovoltaic plants in a grid section deliver electricity at the same time. Market participants react to weather forecasts, price forecasts and grid restrictions. Good forecasts therefore have their own economic value. They reduce balancing costs, improve the dispatch of flexible plants and make unnecessary safety margins superfluous.

Spatial distribution offsets part of the fluctuations. Individual plants fluctuate strongly, large fleets less so, because the weather is not the same everywhere at the same time. Offshore wind, onshore wind and photovoltaics have different generation profiles: solar power is higher during the day and in summer, wind power often in the winter half-year. Between the coast and inland areas, or between northern and southern Germany, fluctuations also partly balance out. This smoothing does not replace safeguards for dark doldrums (Dunkelflauten) or winter high-load periods, however. Large-scale weather situations can affect many plants at the same time.

Residual load, flexibility and storage

The significance of fluctuating generation is most evident in the residual load. This is the electricity demand that, after deducting feed-in from wind and photovoltaics, still has to be met by other sources or through imports. When a lot of wind and solar power is available, the residual load falls. If necessary, it can even become negative. When little is available, it rises. For operation, therefore, it matters not only how much electricity is consumed but when, in relation to weather-dependent feed-in.

Flexibility can come from several sides:

  • Controllable power plants can increase or reduce their output.

  • Storage units absorb electricity and release it again later.

  • Consumers such as heat pumps, electric vehicles, electrolysers or industrial processes can shift their demand in time if technical equipment, price signals and contracts allow it.

  • Grids and interconnectors enable balancing across larger areas.

None of these options alone covers all requirements. Fluctuations within minutes require different means than periods of little wind and sun lasting several days. A local grid bottleneck is a different problem from a national capacity shortfall.

In debates, storage is often regarded as a simple answer to fluctuating generation. Technically, what matters is which periods are to be bridged. Battery storage is well suited to minutes, hours and balancing within a day. Pumped storage fulfils similar tasks but is tied to suitable sites. Hydrogen, synthetic gases or other long-term storage can close gaps lasting several days or seasons. However, they have significantly higher conversion losses and need their own infrastructure. Storage also does not automatically solve grid congestion. This applies, for example, if it is connected at the wrong place or is operated according to market prices differently from what would help the grid.

Market prices, curtailment and grid congestion

Fluctuating generation has very low variable costs because hardly any fuel costs arise. Wind and solar power therefore usually enter the merit order of the electricity market with low bids. With high feed-in, wholesale prices can fall sharply or turn negative. This happens above all when inflexible generation, support rules, technical minimum loads, grid congestion and low demand coincide. Negative prices are therefore not simple proof of too much renewable energy. Rather, they show that, in a particular market and grid context, absorption capacity is lacking.

In Germany, support reacts to this. Under section 51 of the EEG 2023 (Renewable Energy Sources Act) in the version applicable since 25 February 2025, the value to be applied (anzulegender Wert) falls to zero as long as the spot market price is negative. For the plants concerned, there is then neither market premium nor feed-in tariff. Smaller plants are partly exempt: plants below 100 kilowatts until the end of the year in which a smart metering system is installed, and plants below 2 kilowatts until a determination by the Bundesnetzagentur (Federal Network Agency). For older plants, transitional rules apply with minimum durations of negative prices, depending on commissioning or award. Under section 51a EEG, the remuneration period is extended by the periods concerned. These rules are intended to create incentives to feed in less during surpluses.

Curtailment means that plants feed in less electricity than the weather would allow. Causes can be grid congestion, local overloads, system security requirements or economic decisions at negative prices. Previously, grid-related curtailment of renewable and CHP plants in Germany was regulated as feed-in management (Einspeisemanagement) under sections 14 and 15 EEG. These special rules ceased to apply on 1 October 2021. Since then, grid-related curtailment has been part of the unified redispatch under sections 13 and 13a EnWG (Energy Industry Act), colloquially Redispatch 2.0. In principle, generation plants and storage units from 100 kilowatts as well as remotely controllable smaller plants are included, and distribution system operators also carry out measures. The term feed-in management therefore no longer correctly describes the current legal situation.

Priority feed-in for renewable and CHP electricity continues to apply in redispatch but is made concrete through minimum factors. If curtailing conventional generation is also suitable for relieving a bottleneck, grid operators may, in principle, only resort to plants with priority if curtailing them is many times more effective. Curtailment is not waste in a moral sense. It is an operational measure for cases where transport, consumption, storage or conversion are not available in time. At the same time, it is a cost signal: if a lot of energy is regularly curtailed, suitable grids, flexible demand, storage, appropriate market rules or a spatially better coordinated expansion are lacking.

Conflicts arise where technical possibility, market rule and responsibility diverge. A wind farm can generate electricity and the market price can suggest operation, while the local grid is overloaded. Different actors are responsible for expansion, connection, financial compensation and redispatch. Anyone who wants to understand the effect of fluctuating generation must therefore consider, besides the plant, also balancing, grid connection, congestion management, support mechanisms and bidding zones.

Typical misunderstandings

First: that fluctuating generation is fundamentally unreliable. The individual plant is weather-dependent, but the plant fleet can be described statistically and largely forecast. Security of supply does not depend on whether every type of generation can supply alone at any time. What is decisive is whether the entire system of power plants, grids, storage and consumers covers the load in every hour. Wind and sun change the requirements for this system but do not make it superfluous.

Second: that high annual generation means firm capacity. A country can obtain a large share of its annual electricity from wind and photovoltaics and still need additional capacity for certain hours. This can come from storage, flexible demand, controllable power plants, imports or other sources. The actual planning question is: which combination delivers the necessary security, at what cost and with what dependencies?

Third, costs: fluctuating generation is sometimes assessed only by the plant's generation costs, sometimes only by the integration costs of the overall system. Either alone distorts the picture. The low levelised costs of electricity of wind and photovoltaics are real. So are costs for grids, balancing, reserves, digitalisation, storage and flexibility incentives. Conversely, conventional systems also cause costs for fuels, emissions, capacity provision, grid stability and import dependencies. A robust comparison must disclose the system boundary.

Conclusion

Fluctuating generation describes the weather-dependent availability of electricity from wind and photovoltaics. The term explains why a power system with high shares of these sources must be organised more strongly around time profiles, forecasts, residual load, grid capacities and flexibility. It does not, however, explain security of supply, market prices or system costs on its own. For that, one must consider the rules and technical functions that turn fluctuating feed-in into a reliable electricity supply.

Sources

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Edited by eCMS on 12.11.2026 05:05

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