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Glossary · Energy industry & energy transition

Curtailed energy (Ausfallarbeit)

Lost energy Curtailment Feed-in Management redispatch renewable energies grid congestion compensation

Curtailed energy (Ausfallarbeit) is the quantity of electrical energy that a generation or storage facility would presumably have fed in without a grid-related intervention, but did not feed in because of that intervention. Since October 2021, congestion management in Germany has been carried out through redispatch. There, the Bundesnetzagentur defines the term precisely: curtailed energy is the difference between the theoretical generation of a technical resource and the value of the power limit specified by the measure. In the case of negative redispatch, i.e. throttling, curtailed energy is positive. In the case of positive redispatch, when a facility generates more on instruction, it is negative and is then referred to as additional energy (Mehrarbeit). A technical defect of the facility is therefore not what is meant. It concerns a change in quantity triggered by an instruction or control action of the grid operator.

In the past, the term Ausfallarbeit was used mainly in feed-in management under the EEG when wind and solar facilities were throttled. With the switch to Redispatch 2.0 in October 2021, renewable energy facilities and CHP facilities were also brought into the redispatch regime under the Energy Industry Act (Energiewirtschaftsgesetz). Since then, grid operators have in principle been required to balance the measures in the balancing group accounts.

Unit and calculation logic

Curtailed energy is a quantity of energy. It is stated in kilowatt hours, megawatt hours or gigawatt hours. It must be distinguished from power in kilowatts or megawatts. Calculation is done in quarter-hours: for each quarter-hour of the measure, the difference between theoretical power and the limit is multiplied by one quarter of an hour, i.e. 0.25 hours. An example: a wind turbine could have delivered 8 MW but is only allowed to feed in 3 MW. Per quarter-hour this gives 5 MW × 0.25 h = 1.25 MWh. If the measure lasts two hours, i.e. eight quarter-hours, the total comes to about 10 MWh. This presupposes that the wind and the facility would actually have delivered 8 MW the whole time.

An upper limit always applies: the theoretical power may exceed neither the rated power nor the power actually usable. If part of the facility is unavailable due to maintenance, or if the operator has throttled the facility for market reasons, the lowest of these values applies in each case. If, for example, curtailment takes place anyway in the case of negative electricity prices, no curtailed energy arises to that extent. The same applies to standstill due to lack of wind, icing or a fault: this is an ordinary generation loss, not curtailed energy.

Distinction from curtailment and redispatch

Curtailment is the intervention itself, i.e. the limitation of the feed-in power, for example to 60 percent of installed capacity. Curtailed energy is the quantitative result over the duration of the measure. Redispatch is the umbrella term for the entire measure: before a bottleneck, feed-in is reduced, and beyond it, generation is increased elsewhere. Curtailed energy concerns only the individual facility whose operating mode was adjusted.

How curtailed energy is determined

The energy not generated cannot be measured. It must be reconstructed from measured values and models. Since the determination BK6-23-241 of 7 May 2026, the annex "Balancing of redispatch measures (Bilanzieller Ausgleich von Redispatch-Maßnahmen, BilAReM)" applies. It replaces the previous determinations BK6-20-059, -060 and -061. For wind and solar facilities there are three settlement variants:

  • Peak-settlement method (Spitzabrechnung): The basis is weather data measured at the facility itself. For wind turbines, wind speed is measured at the nacelle or hub and converted into a theoretical power using the certified power curve. A correction factor reconciles this calculation with the actual feed-in. It is taken from the nearest four consecutive quarter-hours before or after the measure in which the unthrottled facility delivered at least 10 percent of its rated power. For solar facilities, the measured irradiation is set in relation to a reference day without a measure.

  • Simplified peak-settlement method (vereinfachte Spitzabrechnung): The calculation is carried out as in the peak-settlement method, but using weather data from a meteorological service provider or measured values from a suitable nearby reference facility. For solar facilities, for example, satellite data evaluated using the Heliosat-2 method are considered suitable. If the operator does not choose a variant, this one is applied.

  • Flat-rate settlement (Pauschal-Abrechnung): For wind, the last fully measured, unthrottled quarter-hour value before the measure is carried forward. For photovoltaics, the installed capacity is multiplied by a fixed facility factor, for example 0.6189 in summer between 9 a.m. and 3 p.m. (UTC+1). A 1,000 kW facility that is limited to 300 kW in this time window thus arrives at about 319 kW × 0.25 h ≈ 80 kWh of curtailed energy per quarter-hour. The flat rate is now open only to facilities that were already assigned to it, and only until the end of 2028. Facilities without quarter-hourly metering are excepted: for them, flat-rate settlement continues to apply.

Operators of offshore wind farms can additionally choose the wind-bin method. Here, correction factors are formed separately by wind speed ranges 0.5 m/s wide, and a loss factor represents the losses within the wind farm. For facilities with non-fluctuating generation, such as biomass or CHP, either the submitted schedule is used (peak settlement) or the last quarter-hour before the measure is carried forward (flat-rate settlement). New is a rule for oversubscribed connections: if more facilities are installed at one grid connection than the connection capacity can accommodate, the curtailed energy is reduced to what could have been fed in via that connection at all. By the end of the third month after the measure at the latest, the curtailed energy must have been agreed between the parties involved or the dissent must have been recorded.

Balancing and financial compensation

Under § 13a EnWG, operators of facilities with a rated power of 100 kW or more, as well as facilities that can be remotely controlled at any time, must carry out or tolerate adjustments at the request of the grid operator. They are entitled to reasonable financial compensation for this. It is reasonable if, taking the balancing compensation into account, it leaves the operator economically neither better nor worse off than without the measure. For renewable energy and CHP facilities, the compensation covers lost revenue and additional expenses; saved expenses are deducted. Curtailed energy is thus the quantity basis for the compensation payment.

Whether the grid operator also balances the missing quantity in the balancing group accounts depends on the balancing model. In the schedule-value model (Planwertmodell), the grid operator books the balancing quantity into the affected balancing group on the basis of the planned operating mode. If the curtailed energy determined later deviates from this, the difference is settled financially at the index balancing energy price (Index-Ausgleichsenergiepreis). For facilities in the forecast model (Prognosemodell), which mainly concerns facilities on distribution grids, no balancing by the grid operator takes place until 31 December 2031. However, the curtailed energy is still determined, for example as a settlement basis for direct marketers. Facilities that are important for resolving congestion in the transmission grid are to switch to the schedule-value model by the beginning of 2031.

Orders of magnitude

According to the Bundesnetzagentur, about 3.5 percent of renewable electricity generation was curtailed in 2024 because of grid congestion. The Renewable Energies Agency (Agentur für Erneuerbare Energien) puts the quantity at around 9.4 billion kilowatt hours. Wind, offshore and onshore, was mainly affected; curtailment of photovoltaics roughly doubled to 1,389 GWh. Operators received around 554 million euros in compensation for this. About 47 percent of curtailments affected facilities in the distribution grid, but the underlying congestion was around 74 percent in the transmission grid. These figures apply to Germany and to 2024. They fluctuate strongly with the wind year, capacity additions and grid expansion, and for later years should be looked up in the current reports.

Classification and typical misunderstandings

"Wasted electricity" is a catchy but imprecise expression. Physically, nothing was destroyed: wind or solar radiation was simply not converted into fed-in electricity. Curtailed energy is also a calculated quantity. The same measure can lead to different results depending on the settlement variant, weather data and correction factor. That is why there are binding formulas, deadlines and clearing processes.

A high level of curtailed energy first of all indicates a gap between available generation and grid capacity. It does not yet say which countermeasure is the right one. Options include grid expansion, storage, switchable loads or more flexibility. Which solution is worthwhile depends on the location, frequency and duration of the congestion. A small residual quantity can be economically more sensible than a grid expansion that would be needed only for a few hours a year. For an assessment, one therefore needs ratios, such as the share of renewable generation, and a comparison with the costs of the alternatives.

Sources

Related glossary entries

Edited by eCMS on 11.11.2026 14:37

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