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Natural patterns like seasons (yearly), the variations in weather (weekly), day and night and our rhythm of waking up, going to work, coming home, etc. cause both the need for energy and the availability of energy to fluctuate. Flexibility is about balancing energy supply and demand on all of these timescales. In the ETM you have the ability to choose and adjust how you deal with these fluctuations in the 'Flexibility' section. On this page, concepts relevant for understanding flexibility are defined.

Flexibility for different types of energy fluctuations

A first distinction to make when looking into flexibility, is to decide whether it is required for longer or for shorter timescales. For longer timescales, the amount or volume of energy supplied or used is more interesting. For shorter timescales, the ability to produce or use a certain amount of energy is more interesting. This is called capacity. For a typical energy system fluctuations occur on both timescales, and enough flexibility should be provided for both volume and capacity.

Some forms of flexibility are more suited to process large (fluctuations in) volumes of energy, others are more suitable for large fluctuations in capacity. Only some are suited to both. Some examples are:

Suitable for large (fluctuations in) volume

  • Imports/exports of gas/hydrogen
  • Power-to-gas: hydrogen production from electricity
  • Storage of gas/hydrogen
  • Seasonal storage of heat

Suitable for large or sudden (fluctuations in) capacity

  • Storage in lithium-ion batteries
  • Dispatchable power and heat plants
  • Demand side response

Suitable for both volume and capacity

  • Imports/exports of electricity
  • Power-to-heat: heat production from electricity (with heat storage)
  • Curtailment of renewable electricity production
  • Large-scale electricity storage: pumped hydro storage

Definitions of flexible and inflexible supply and demand

The different forms of flexiblity listed above indicate that there are also types of supply and demand that are inflexible. In the ETM this distinction between inflexible and flexible supply and inflexible (or baseload) and flexible demand is explained in the section about 'Hourly inflexible supply and demand':

  • Inflexible supply: this is the energy produced by technologies that cannot be regulated easily by humans or that run continuously based on cost considerations. Wind and solar are good examples of technologies that cannot be regulated by humans. They do show variable behaviour (volatile changes due to environmental effects) but they for instance do not follow hourly electricity prices. They are therefore called inflexible. Nuclear power plants are an example of power plants that can be adjusted to run continuously based on cost considerations. These kind of plants are called must-run.
  • Flexible supply: this energy production typically does follow man-made rules (such as hourly electricity prices) and includes so-called dispatchable plants like gas-fired power plants.
  • Inflexible (or baseload) demand: this type of energy consumption is considered fixed because it cannot be regulated easily. This for example includes industrial processes that need to run continuously, or consumption that does not respond to hourly electricity prices such as individual households. Most of the final electricity demand of sectors falls into this category.
  • Flexible demand: this is energy consumption that can be increased, reduced, or shifted in time if needed.

In the ETM, a systems perspective is used to classify technologies as flexible or inflexible. This means that when a technology is considered flexible for a specific energy carrier, it is also considered flexible for all energy carriers. This distinction is relevant for conversion technologies, such as power-to-heat. Power-to-heat technologies use electricity to produce heat, depending on whether the hourly electricity price is low enough. From the perspective of the energy carrier electricity, power-to-heat is therefore considered a flexible technology because it follows man-made rules. Because the systems perspective is used, power-to-heat is then also considered a flexible technology for the energy carrier heat - even though the technology follows no man-made rules for the district heating network.

Note: the classification of technologies as flexible or inflexible, is based on their representation in the ETM. Some technologies that may be flexible in reality, can be modelled in such a way that they are considered inflexible by the ETM. For example, the import of natural gas can be considered flexible in reality, but in the ETM it is represented by a flat curve and therefore seen as inflexible.

The need for flexibility

In the ETM, we have made selection of charts available that can give you insight in the need for flexibility in the energy system. These charts are intended to give you a sense of the mismatch between supply and demand on various timescales, for the energy carriers electricity, gas, hydrogen, and heat for district heating. These charts are accompanied by explanatory texts that build your understanding of the need for flexibility in a logical order. If you are interested in exploring the need for flexiblity in your scenario, we therefore recommend starting at the 'Overview'.

Keep in mind that the definitions used to classify flexible and inflexible technologies described above, affect what is shown in the charts. In the paragraph below you can find a complete overview of which technologies are flexible, and which are inflexible, for the four energy carriers defined above.

Categorization of flexible and inflexible technologies


See the Flexibility → Electricity storage and Flexibility → Electricity conversion sections of the model.


  • Inflexible:
    • Must-run / volatile: wind turbines, solar panels, hydro power, must-run nuclear plants
  • Flexible:
    • Dispatchable power plants
    • Batteries discharging: household batteries, vehicle-to-grid, large-scale batteries, etc.
    • Import


  • Inflexible (baseload):
    • Final electricity demand in sectors
    • Must-run heat pumps / boilers for district heating
  • Flexible:
    • Batteries charging: household batteries, vehicle-to-grid, large-scale batteries etc.
    • Conversion: power-to-hydrogen, power-to-heat (for industry or district heating)
    • Curtailment
    • Export

Flexible demand technologies are governed by a "willingness to pay": a maximum price they are willing to pay to consume electricity. Whenever the spot price of electricity exceeds this threshold, the they will stop consuming electricity. See our documentation on electricity storage for more information.



  • Inflexible:
    • Production green gas and LNG (flat curve)
    • Extraction natural gas (flat curve)
    • Import of natural gas (flat curve; constant import of gas to balance yearly production of gas)
  • Flexible:
    • Gas from storage (in the ETM, gas is automatically buffered throughout the year)


  • Inflexible (baseload):
    • Final gas demand in sectors
    • Export of gas (flat curve; constant export of gas to balance yearly production of gas)
    • Distribution losses
  • Flexible:
    • Gas used in dispatchable power plants and heat boilers for district heating
    • Gas entering storage (in the ETM, gas is automatically buffered throughout the year)


See the Supply → Hydrogen section of the model.


  • Inflexible:
    • Must-run / volatile: dedicated offshore wind turbine or solar PV plant for H2, steam methane reforming, biomass gasification
    • Import of hydrogen (flat curve; constant import of hydrogen to balance yearly production of hydrogen)
  • Flexible:
    • Hydrogen from storage (in the ETM, hydrogen is automatically buffered throughout the year)
    • Hydrogen produced by power-to-gas


  • Inflexible (baseload):
    • Final hydrogen demand in sectors
    • Export of hydrogen (flat curve; constant export of hydrogen to balance yearly production of gas)
    • Distribution losses
  • Flexible:
    • Hydrogen used in dispatchable power plants and heat boilers for district heating
    • Hydrogen entering storage (in the ETM, gas is automatically buffered throughout the year)

Heat for district heating

See the Supply → District heating section of the model.


  • Inflexible:
    • Must-run / volatile: solar thermal, residual heat from industry, geothermal heat
    • Import of heat (flat curve)
  • Flexible:
    • Heat produced by power-to-heat
    • Heat produced by CHPs, as CHPs participate as dispatchable power plants in the electricity merit order
    • Dispatchable heat sources: collective heat pump, hydrogen heater, etc.
    • Heat from seasonal storage


  • Inflexible (baseload):
    • Final heat demand in sectors
    • Losses: distribution losses, heat surplus (wasted)
  • Flexible:
    • Heat entering seasonal storage