Flexibility in this case refers to power system operation, specifically as follows from the National Renewable Energy Laboratory website:

“Flexibility of operation—the ability of a power system to respond to change in demand and supply—is a characteristic of all power systems. Flexibility is especially prized in twenty-first century power systems, with higher levels of grid-connected variable renewable energy (primarily, wind and solar).” (National Renewable Energy Laboratory: 21st Century Power Partnership, May 2014)

Per EnergyPedia.info: As the world shift to more renewable energy, especially variable ones such as wind and solar, a paradigm shift in the power sector has gradually taken place to meet the transition. In particular is the growing trend of more focus on the so-called “Power System Flexibility” in the academic and industrial sectors in this field.

According to the International Energy Agency, the flexibility of a power system refers to “the extent to which a power system can modify electricity production or consumption in response to variability, expected or otherwise”[1]. Another source described it as “the modification of generation injection and/or consumption patterns in reaction to an external signal (price signal or activation) in order to provide a service within the energy system” [2].

Flexibility can therefore also refer to the capability to change power supply/demand of the system as a whole or a particular unit (eg. a power plant or a factory). (EnergyPedia)

Without sufficient flexibility, system operators may need to frequently curtail (decrease the output of) wind and solar generation. Although low levels of curtailment (e.g., less than 3%) may be a cost-effective source of flexibility, significant amounts of curtailment can degrade project revenues and contract values, impact investor confidence in renewable energy revenues, and make it more difficult to meet emissions targets. (From NREL)

Fossil Fuels

Fossil fuels refer to energy sources, such as oilcoal and natural gas, which are non-renewable resources that were formed when prehistoric plants and animals died and subsequently buried by layers of rock. (US Dept of Energy)


Describes the rate per second at which electrical current changes direction or alternates between positive and negative voltage. It is measured in hertz (Hz), an international unit of measure where 1 hertz is equal to 1 cycle per second.

The US standard power frequency is 60 Hertz. In other parts of the world, 50 Hertz is used. The frequency for all types of power generation needs to be at the standard to keep on our lights.

If the different generators don’t spin at the same speed, the system becomes unstable. If there is more demand for electricity than there is supply — frequency will fall. If there is too much supply the frequency will rise. Increases or decreases in power frequency as little as one percent will put equipment and power infrastructure at risk of damage.

Learn more about frequency at these links: Penn State College of Earth and Mineral Sciences and Medium.com