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Glossary · Energy industry & energy transition

Yield (Energy Yield)

yield electricity generation photovoltaics wind energy Energy Kilowatt hour performance ratio

In the power system, yield refers to the amount of energy that a plant, a power plant fleet or a generation technology actually generates or makes usable within a given period. It is usually stated in kilowatt-hours, megawatt-hours or gigawatt-hours. The installed capacity describes the maximum electrical power a plant can provide under defined conditions. Yield, by contrast, describes how much energy has actually been produced. A simple analogy: power corresponds to the speed of a car, yield to the distance it has covered by the end of the day.

For photovoltaic plants, yield is the electrical energy generated in the period under consideration, for example per day, month or year. The same applies to wind turbines. How much electricity is produced there from the installed capacity depends strongly on the wind conditions at the site, as well as on hub height, rotor area and technical availability. For dispatchable power plants, the weather plays a smaller role. More important there are fuel availability, dispatch planning, market prices, maintenance, technical minimum loads and regulatory requirements.

The unit is central to understanding. A kilowatt-hour is a quantity of energy. A plant with a power of one kilowatt generates exactly one kilowatt-hour in one hour only if it operates at one kilowatt for the entire hour. A photovoltaic plant with ten kilowatts of installed capacity therefore does not automatically generate ten kilowatt-hours per hour. Its instantaneous power changes with irradiance, temperature, orientation, shading and inverter operation. Yield is the sum of this varying power over time, mathematically the integral of power.

Distinction from power, capacity utilisation and revenue

Yield is often confused with installed capacity, which leads to false expectations. Two plants of equal capacity can deliver very different annual yields if they are located at different sites, are oriented differently or stand idle with different frequency. A wind turbine at a windy coastal site can generate significantly more electricity from the same rated power than a turbine at a weaker inland site. A photovoltaic plant on an east-west roof has a different yield profile than a plant optimally oriented to the south. With the same capacity, it usually also delivers a lower annual yield, but this is distributed more evenly over the day.

A related term is full-load hours. They indicate how many hours a plant would theoretically have to run at full power to generate its actual annual yield. A plant with 1 megawatt of capacity and an annual yield of 2,000 megawatt-hours arrives at 2,000 full-load hours in calculation terms. A year has 8,760 hours. This would correspond to an average utilisation of just under 23 percent, referred to as the capacity factor. The indicator links power and yield. However, it says nothing about when the electricity was generated and what value it had in the grid or on the market.

Physical yield must be distinguished from financial revenue. A high electricity yield does not automatically lead to high income. Revenue depends on the prices at which the electricity is sold, whether it is subsidised or self-consumed, whether grid congestion occurs and which marketing form was chosen. A renewable energy plant can generate a lot of electricity and still achieve only low market revenues during hours of low or negative prices. For the time-based valuation, there is the term market value. Physical and economic yield diverge as soon as time, location and price formation are taken into account.

Gross yield, net yield and specific yield

In practice, it must be clarified which system boundary is meant. Gross yield is usually the energy generated before certain losses. Net yield can mean the energy after inverter losses, plant auxiliary consumption, transformer losses or other deductions. There is no uniform definition, so the reference level should always be stated. For grid-connected plants, it is also important whether total generation is measured or only the quantity fed into the grid. If part of the electricity is used directly in the building as self-consumption, the actual yield is higher than the quantity visible at the grid connection point.

To compare photovoltaic plants, the specific yield is frequently used, stated in kilowatt-hours per kilowatt-peak (kWh/kWp). Kilowatt-peak is the module power under standard test conditions: 1,000 watts of irradiance per square metre at a cell temperature of 25 degrees. A specific annual yield of 1,000 kWh/kWp means that the plant generated 1,000 kilowatt-hours per installed kilowatt-peak in the year. Numerically, the specific annual yield corresponds to the full-load hours. Fraunhofer ISE treats both terms as synonymous. It defines them as annual generation divided by the rated power, explicitly referring there to the AC side. Because many figures instead use the DC module power, the reference quantity should be checked when making comparisons.

On the order of magnitude in Germany: a Fraunhofer ISE study assumes a horizontal annual irradiation of about 950 to 1,300 kilowatt-hours per square metre, depending on the region. After deducting losses in the plant, this results in average annual yields of about 935 to 1,280 kWh/kWp with favourable orientation. East-west orientation or a flatter mounting angle lowers these values. According to Fraunhofer ISE, the regional difference in irradiation does not translate one to one into the specific yield, because factors such as module temperature, soiling and snow cover play a part. The indicator allows comparisons between plants of different sizes, but does not replace a site analysis. A technically flawless plant can have a low specific yield if the roof is unfavourably oriented or regularly shaded.

Another indicator is the performance ratio. It relates the actually measured yield of a photovoltaic plant to the yield the plant would achieve without any losses at the measured irradiance on the module plane. In the monitoring standard IEC 61724-1, this corresponds to the quotient of final yield and reference yield. The difference from 100 percent sums up all losses, including temperature, inverter and cable losses, soiling, shading and downtimes. Because the local irradiance is in the denominator, the performance ratio helps to separate technical quality from site conditions. A sunny site can deliver high absolute yields despite mediocre plant technology. A very good plant at a less sunny site generates less electricity in absolute terms, but can achieve a high performance ratio. In summer, the indicator is typically somewhat lower than in winter because of higher module temperatures. It is therefore usually evaluated over a whole year.

Why the timing of yield matters

For the power system, the annual quantity alone is not sufficient. Electricity must be generated, or supplied from storage, exactly when it is consumed. The yield of a plant therefore has two dimensions: the quantity of energy and the time profile. Photovoltaics delivers a lot of electricity in bright daytime hours and none at night. Wind energy fluctuates with weather conditions and can deliver very high or very low yields over hours or days. These profiles influence the residual load, storage demand, grid loading, balancing energy demand and market prices.

A gigawatt-hour of annual yield is worth different amounts to the system, depending on whether it occurs in hours of high demand and scarce generation or in hours with plenty of electricity in the grid anyway. Yield in surplus hours is nevertheless not worthless. It can charge storage, operate electrolysers, generate heat or supply flexible industrial processes. For this, however, plants, market rules and grid connections must be organised so that such use is possible. If suitable flexibility is lacking, additional yield is temporarily curtailed.

Curtailment reduces the usable yield. If a wind or solar plant is allowed to feed in less because of grid congestion, a gap arises between possible and actual yield. In Germany, this takes place within the framework of grid congestion management (redispatch). According to the Bundesnetzagentur, in the third quarter of 2025 around 2,411 gigawatt-hours from renewable plants were curtailed, about four percent of renewable generation. Just under half of this concerned photovoltaics, and a growing share of the congestion occurred in the distribution grid. Plants in direct marketing are, in the event of such interventions, economically placed largely as if the curtailment had not taken place. Physical yield loss and financial loss are therefore not the same thing. For the assessment of sites, grid expansion and market rules, the difference nevertheless matters. A very windy site can be less attractive from a system perspective if its grid connection is regularly limited. A plant with lower theoretical potential at a grid-friendly site can, by contrast, deliver more usable yield.

Yield forecasts and their limits

Yields are frequently forecast in advance. For photovoltaics, irradiation data, roof pitch, orientation, shading, module type, inverter design and historical weather data are used. For wind energy, wind measurements, long-term correlations, turbulence, terrain structure and power curves of the turbines are evaluated. Such forecasts are necessary for financing, planning, tenders and the assessment of grid connections. They nevertheless remain probabilistic statements. In expert reports, they are therefore often stated with exceedance probabilities, for example as P50 or P90. A P90 value is the yield that is reached or exceeded with 90 percent probability. A single year can deviate significantly from the long-term average.

This uncertainty is often underestimated. A low annual yield is not automatically a technical fault. It can result from a weak wind year, low solar irradiation, long snow cover, frequent grid congestion or extended maintenance periods. Conversely, a favourable weather year can lead to an overestimate of a plant's long-term performance. Robust assessments require multi-year data, availability analyses and weather-adjusted comparisons. For photovoltaics, the performance ratio is better suited for this than the absolute yield.

Who uses the figure also makes a difference. Operators are interested in the billable yield and direct marketers in the time-valued electricity. Grid operators pay attention to feed-in profiles and congestion effects, banks to the expected annual production and default risks. The same physical term thus appears in different decision contexts. The debate becomes imprecise when conclusions about security of supply, economic viability or grid compatibility are drawn from a single yield figure without naming the system boundary.

Conclusion

Yield shows how much energy a plant actually provides. On its own, however, it does not say whether this energy is available at the right time, in the right place and under economically viable conditions. For the assessment of electricity generation, power, yield, yield profile, grid connection, curtailment, market value and flexibility should therefore be considered together. The term is precise when it denotes a measured or expected quantity of energy with a clear time span, a clear system boundary and a named reference quantity.

Sources

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Edited by eCMS on 11.11.2026 19:47

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