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Glossary · Energy industry & energy transition

Full Load Hours (Volllaststunden)

full load hours Utilisation electricity generation power Energy Capacity factor renewable energies

Full load hours (in German: Volllaststunden) indicate for how many hours a power generation plant would theoretically have had to run at its rated capacity to reach the amount of energy it actually generated in a given period. Usually a calendar year is considered. The indicator links power and energy. Power describes how much a plant can deliver at a given moment. Energy describes the amount generated over time.

The calculation is simple: the electrical energy generated, for example in megawatt hours, is divided by the installed capacity, for example in megawatts. A wind turbine with a rated capacity of 5 megawatts and an annual generation of 15,000 megawatt hours thus reaches 3,000 full load hours. This does not mean that it ran at exactly 5 megawatts for 3,000 hours. Its feed-in fluctuates with the wind. Full load hours condense this varying operating behaviour into a theoretical comparison figure. A vivid image: one mentally pushes the entire annual generation together into a single block in which the plant runs continuously at full load. The length of this block is the full load hours.

The unit is hours per period, usually hours per year. A normal year has 8,760 hours, a leap year 8,784. Dividing the full load hours by the number of hours in the period gives the capacity factor. In a normal year, 3,000 full load hours correspond to about 34 percent (3,000 divided by 8,760). Internationally the capacity factor is more common, whereas in the German-speaking energy sector full load hours are more familiar. Both figures describe the same relationship as long as the period and the capacity basis are the same. For plants that were only built during the course of the year, one must also determine whether the capacity at year-end or a time-weighted annual average is used. Using the capacity at year-end would make the full load hours come out too low in periods of strong capacity additions.

Distinction from operating hours, availability and utilisation

Full load hours are often confused with operating hours. Operating hours count how long a plant was switched on or in operation. A wind turbine can run for many hours in weak wind and generate only a small part of its rated capacity. Conversely, a gas turbine may run only a few hours a year and be close to its maximum output during that time. Full load hours therefore do not measure the duration of operation, but the amount of energy generated in relation to capacity.

Availability is also something different. A plant can be technically available and still not generate electricity: because there is no wind or no sun, because the market price is too low, or because the grid operator limits feed-in. Availability describes the ability to run when needed. Full load hours show the result of resource, technology, market conditions, grid restrictions and mode of operation.

The term utilisation is used inconsistently. Sometimes it means the capacity factor, sometimes the economic use of a plant, sometimes the load on the grid. Full load hours are more precise because the calculation rule is fixed. They are only comparable, however, if it is clear which capacity they refer to: gross or net capacity, generator capacity, inverter capacity or installed module capacity.

In photovoltaics this distinction is particularly important. Module capacity is stated in kilowatt peak under standardised test conditions. How much actually flows into the grid additionally depends on inverters, orientation, shading, temperature, self-consumption and curtailment. Full load hours based on module capacity turn out differently from values based on maximum inverter capacity. In photovoltaics, the figure based on module capacity is often called specific yield and is stated in kilowatt hours per kilowatt peak. Numerically it corresponds to the full load hours. For comparisons, the reference basis and the measuring point must always be stated.

Typical orders of magnitude in Germany

Full load hours translate installed capacity into expected amounts of electricity. 10 gigawatts of photovoltaics, 10 gigawatts of offshore wind and 10 gigawatts of gas-fired power plants deliver very different annual amounts. According to analyses by the Energy-Charts of Fraunhofer ISE, average full load hours in Germany between 2022 and 2025 were about 915 hours for photovoltaics, 1,765 hours for onshore wind and 2,895 hours for offshore wind. Nuclear power plants reached around 7,600 hours between 2015 and 2022. These values are averages across the entire plant fleet. They fluctuate noticeably depending on the weather year: 2021 was a weak wind year, and the values for onshore and offshore wind were clearly below the long-term average.

Individual sites and new plants can deviate considerably from these fleet averages. The Fraunhofer study on levelised cost of electricity assumes, for new onshore wind turbines in Germany, sites with between about 1,800 and 3,200 full load hours, and for offshore wind farms about 3,200 to 4,500 hours. For photovoltaics, the assumed yields rise from northern to southern Germany, from around 950 up to about 1,300 kilowatt hours per kilowatt peak. According to analyses by Fraunhofer IEE, individual good offshore wind farms reached on average more than 4,000 full load hours. There are several reasons why the fleet average for photovoltaics is lower: many plants are older, not optimally oriented or partly shaded, and self-consumption can only be estimated statistically.

Conventional power plants can technically reach very high full load hours, but are not necessarily operated that way. Their dispatch depends on fuel and CO₂ costs, electricity prices, maintenance periods, the rest of the power plant fleet and demand. For them, full load hours are therefore above all a result of the market and not a fixed technical property.

Economic significance

Many costs of a plant are fixed or depend on capacity. The investment is incurred per kilowatt of installed capacity. The more kilowatt hours a plant generates over its lifetime, the more these fixed costs are spread out. Full load hours therefore have a great influence on the levelised cost of electricity. If the same wind turbine is located at a site with more full load hours, it generates electricity at lower cost per kilowatt hour. Photovoltaics achieves comparatively few full load hours in Germany, but is nevertheless competitive because of low investment costs.

A different logic applies to flexible power plants. A peak-load power plant with few full load hours is not automatically inefficient or useless. It is needed precisely because it provides capacity in scarce hours. Its revenues then come not from many kilowatt hours generated, but from high prices in scarcity situations, from balancing energy markets or from other mechanisms that secure capacity. Here, full load hours describe the amount of energy, but not the value of the plant for the system.

Typical misinterpretations

  • High full load hours are regarded as a sign of good technology, low ones as a sign of poor technology. Different plants, however, fulfil different tasks. Baseload power plants were designed for long annual operating times, wind and solar plants follow natural resources, and reserve power plants safeguard against rare bottleneck situations. For these technologies, the same indicator does not measure the same function.

  • Installed capacity is offset one to one. Anyone who says that a certain amount of renewable capacity replaces the same amount of conventional capacity overlooks full load hours, feed-in profile and firm capacity. Photovoltaics delivers large amounts of energy over the year, but contributes hardly anything to meeting load on winter evenings. For security of supply, one additionally needs load profiles, the residual load, storage, dispatchable generation, grids and flexible demand.

  • From few full load hours it is concluded that a plant is worthless for the system. The annual value says nothing about fuel costs, emissions or marginal costs. For the market value, what counts is when electricity is generated. Photovoltaics generates during the day, when demand is often higher. With growing capacity additions, however, solar plants feeding in simultaneously depress prices in these hours. This cannibalisation effect lowers their market value. Wind energy delivers large amounts especially in autumn and winter.

  • Curtailment is interpreted as a property of the plant. If wind or solar plants generate less because of grid bottlenecks or negative prices, their actual full load hours fall. Possible and actually fed-in full load hours then diverge. The difference can point to grid connection, grid planning, market rules or support design.

Relationship with market, grid and regulation

Full load hours lie at the interface of technology and organisation. The natural resource determines the possible generation profile, and the plant converts it into electrical power. The grid decides whether the electricity can be transported away, and the market whether the generation makes economic sense. Rules on support, grid connection, balancing group responsibility and congestion management determine which generation actually takes place and is remunerated. In Germany, grid-related curtailment of renewable energy and CHP plants was previously regulated as feed-in management under the EEG (Renewable Energy Sources Act). Since 1 October 2021 it has fallen under the unified regime of redispatch 2.0 under the Energy Industry Act (Energiewirtschaftsgesetz). There, feed-in priority is preserved via minimum factors.

With a growing share of weather-dependent generation, the informative value of the indicator changes. Previously, a power plant was regarded as valuable above all if it could run for many hours a year. In a power system with a lot of wind and photovoltaics, it increasingly matters whether generation and consumption match in time. A plant with many full load hours helps little if it generates mainly in hours with surpluses that are already present anyway. A plant with few full load hours can be valuable if it covers scarcity hours or reduces grid bottlenecks.

Special case: storage

Caution is required with storage. A battery storage system does not generate energy. It takes up electricity and releases it later with losses. It has a capacity in megawatts and an energy content in megawatt hours. The ratio of the two indicates how long it can discharge at full power, for example two or four hours. This storage duration is colloquially sometimes also called full load hours. It is, however, a design parameter and not an annual indicator. If one calculates annual full load hours for a storage system from the energy discharged, these describe throughput and not generation. For annual operation, the number of cycles, efficiency, dispatch strategy and price differences between charging and discharging times are more informative.

Conclusion

Full load hours are a useful indicator if the period, the capacity basis and the system boundary are stated as well. They show how much energy actually arises from installed capacity. They do not, however, explain what a plant contributes to security of supply, what market value its electricity has or what grid costs its location causes. Their strength is the translation of capacity into an amount of energy. As the sole yardstick for the value of a technology, they are not suitable.

Sources

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Edited by eCMS on 12.11.2026 05:05

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