Capacity. System Load. Base load.

Not your everyday household terms. However, the transition to renewables for reliable, cost-effective, and environmentally friendly energy generation, makes it important that we understand the meaning behind power industry terms.

Feel free to recommend other helpful terms on our Path to 100% social media pages.

Describes the challenge utilities meet to maintain a balance between energy consumption and the amount of power being generated. If not, a system imbalance can occur which causes electrical equipment and industrial processes to malfunction, lights to flicker, and can cause damage to sensitive electrical equipment – all of which are things that society cannot tolerate. If the imbalance is significant enough the entire electric grid can fail causing black outs.  More and more utilities are using a combination of conventional generation produced by sources such as natural gas, coal, and nuclear, and renewable sources such as wind, solar, and geo-thermal to achieve this balance.

Here’s a little more context:

“Increased variability from more solar and wind power generation means that conventional generating resources must perform differently. They must increasingly provide necessary capabilities, such as frequency reserve and ancillary services (spinning reserves), which ensure grid stability while ramping up and down in response to variations in the renewable resources.

“The grid-planning models can help to indicate the level of flexible conventional generation that is required to balance various levels of renewables. And although coal plants have some ability to ramp, the majority of this responsibility falls on the shoulders of the gas-fired fleet.” (TransForm, March 2017)


Base Load Generation
Power generation plants that operate continuously throughout the year, except for periodic maintenance or upgrading, to meet the majority of customer demand. Utilities traditionally use fossil-fuel generation (natural gas, coal, nuclear) to meet the base level of energy demand because of its reliability.

The baseload they produce is “The average amount of electric power that a utility company must supply at a given time.” (PG&E)


This is a category of sources for power generation derived from biomass (crops like corn) or waste feedstocks (think manure). An example is ethanol. Another is biodiesel which is “A fuel made from vegetable oils, animal fats or recycled grease that can be used instead of petroleum-derived fuel.” (PG&E)


“The amount of electricity a generator can produce when it’s running at full blast. This maximum amount of power is typically measured in megawatts (MW) or kilowatts and helps utilities project just how big of an electricity load a generator can handle.” (US Department of Energy, March 2018)

Let’s be clear—Capacity is not the same level as actual electricity generated, because plants don’t run all the time.

For example, they are taken offline for maintenance or refueling during which time they are not generating power. This is true for all types of generation. For renewables like wind and solar, actual generation also is affected by current weather conditions. For hydro generation, water flows affect generation levels. Extreme cold can affect fossil fuel plants, also, due to ice accumulation on pipes.


Capacity Factor
Capacity factor is a measure of how much energy is produced by a plant compared with its maximum output. It is measured as a percentage, generally by dividing the total energy produced during some period of time by the amount of energy the plant would have produced if it ran at full output during that time. “(NREL Sept 2013)

“Capacity factors allow energy buffs to examine the reliability of various power plants. It basically measures how often a plant is running at maximum power. A plant with a capacity factor of 100% means it’s producing power all of the time.” (US Department of Energy, March 2018)

For example, in the 2016 Integrated Resource Plan Update for Tucson Electric Power (TEP) and UNS Electric, Inc. (UNSE), the Annual Capacity Factor for Solar Photovoltaic fixed installations was 17%. For Solar PV (single-Axis) it was 24%, and for Solar CSP (Storage) it was 38%. New Mexico Wind was 38%, and Arizona Wind was 30%. In this case, the Solar Concentrated Solar Power (CSP) thermal storage and New Mexico Wind tied for highest producing power at 38% for the year.


The amount of electricity that utility customers are using, or trying to use, at one time. (PG&E) It fluctuates throughout the day and night (see Demand Curve), and utilities must forecast those changes in order to bring more generation and power on the grid just before it is needed or less power when demand dips.


Demand Curve
If you use a line graph to track customer demand in a 24-hour period or throughout the year, it produces a curve showing those times of day or seasons of the year when power demand is up and when it is down. Utilities forecast customer demand for electricity so they can produce or purchase the power to deliver through the grid to meet demand. (EnergyMag)


Distributed Energy
Describes small-scale units of local generation connected to the grid at distribution level….Common examples of DERs include rooftop solar PV units, natural gas turbines, microturbines, wind turbines, biomass generators, fuel cells, tri-generation units, battery storage, electric vehicles (EV) and EV chargers, and demand response applications. These separate elements work together to form distributed generation.” (Arenawire, March 2018) Advanced technology allows for two-way power flow on the grid, so these units can generate power that flows back into the grid for use.


Energy Shifting
This term can be used to describe two things.

  • It is used to describe the process of using of energy storage systems (ESS) to store, say excess solar energy to be distributed at night or when there is a peak in demand on a very hot day. A more in-depth discussion about types of ESS and how they improve the reliability of energy can be found at this link: com.
  • It can describe the move from traditional fossil-fuel generation to renewables for more of our electricity generation. Check out the dynamic graphic at this link NY Times, December 2018 to see each of the US state’s unique energy shift story.


Energy Storage Systems (ESS)
Lots of research is focused on finding effective ways to store electricity produced from renewables so it can be available later when customers need it.

“Energy can be stored using electrical, mechanical, thermal, and chemical storage systems, each with their own benefits and appropriate application….

“Chemical storage systems (excluding batteries) typically uses electrical energy to perform water electrolysis, which produces hydrogen.”…

“Mechanical storage systems operate by converting electrical energy into potential or kinetic energy for storage.” (, Dec 2014)

Examples include a flywheel, compressed air or pumped hydro storage systems.

Batteries are another example, but not necessarily ones that use the same technology as the ones in our TV remotes. “The top energy storage technologies include Lithium Ion, Sodium Sulfur, Metal-Air batteries, along with supercapacitors and hydro pumped power…” (Silicon Valley Innovation Center)


Distributed Generation (DG)
Small-scale generation owned or operated by an end-user to supplement energy needs. Rooftop solar is an example of distributed generation. (AECT)


Fossil Fuels
Fossil energy sources, include oilcoal and natural gas, and are non-renewable resources formed when prehistoric plants and animals died and were gradually buried by layers of rock. (US Dept of Energy)


“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)  For example, conventional power plants need to be flexible enough to power up and down quickly to respond to fluctuations of renewable energy generation.


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—the frequency will fall. If there is too much supply the frequency will rise. Increases or decreases in power frequency as little as one percent puts equipment and power infrastructure at risk of damage. Curious to know more? There’s a great explanation with examples at these links: Penn State College of Earth and Mineral Sciences and


Generation Capacity
“The maximum demand that a given generator or group of generators can meet at a given time. For example, a 1,000 megawatt power plant could meet the demand of 1,000 homes using 1 kW of power simultaneously.” (AECT)


Geo-thermal: “
This is energy available as heat emitted from within the earth’s crust, usually in the form of hot water or steam.” (IEA) In the U.S. geothermal energy is often used at the household or campus level. The unique geology of Iceland allows for 25% of its total electricity production to come from geothermal power facilities. (National Energy Authority of Iceland)


Green Certificates or Renewable Energy Certificates
“A green certificate is a tradable asset which proves that electricity has been generated by a renewable (green) energy source.” (

Owners of renewable generation have one REC for each megawatt of generation. They can keep or sell their certificates. REC purchasers can claim that the energy they used came from a renewable source. Their purchase supports renewable power generation. Check out the EPA video on RECs is at this link: RECs: Making Green Power Possible.


Greenhouse Gases
This refers to “gases that contribute to the greenhouse effect by absorbing infrared radiation (heat).” (IEA)


“Microgrids are localized grids that can disconnect from the traditional grid to operate autonomously. …Microgrids support a flexible and efficient electric grid by enabling the integration of growing deployments of distributed energy resources such as renewables like solar.” (US Dept of Energy)


Peak Demand (Peak Load)
The maximum expected load for a given period of time (i.e. 2-7 p.m. in hot summer states).


Peaking Generation
Generation that only operates at times of high demand, and is not needed at times of low demand. (AECT)


“The electrical energy derived from turbines being spun by fresh flowing water. This can be from rivers or from man-made installations, where water flows from a high-level reservoir down through a tunnel and away from a dam.” (IEA)


“A snapshot of the amount of electric power required to meet customers’ demand at a given time, expressed in kilowatts (kW) or Megawatts (MW).” (AECT)


Non-Fossil Fuel
Fuels that are renewable and/or not formed when prehistoric plants and animals died and were gradually buried by layers of rock. This category of fuels includes renewables like solar, wind, biomass, wave energy, and nuclear energy which is considered nonrenewable.  (US Energy Information Administration)


Peak Demand (Peak Load)
“The maximum expected load for a given period of time….” usually expressed in a 24 hour period. (AECT) In winter months, peak demand is often in the morning when people are preparing to go to work and school and in the evening when they return home from work and school. In summer months, peak demand is often from midafternoon to early evening, when outside temperatures are at their hottest and the air conditioning is always running


This is a semiconductor device that directly converts “solar energy into electricity.” (IEA) It is one of the main processes solar panels use to produce electricity.


Power or Electricity Generation
The process of making electricity from a fuel source such as natural gas, the sun, the wind, water, biofuels, coal, and oil.


Renewable Energy
Energy “that is derived from natural processes (e.g. sunlight and wind) that are replenished at a higher rate than they are consumed. Solar, wind, geothermal, hydro, and biomass are common sources of renewable energy.” (IEA)


Renewable Portfolio Standard (RPS)
“A legislative or regulatory mandate or goal for power generated from renewable sources.” (AECT)


Renewable Volatility
Grid instability that occurs with renewable generation because of its dependence on weather. Advances in technology, energy storage, and grid management are reducing this effect.


Smart Grid or Advanced Metering System (AMS)
“An enhancement to the electric grid that allows customers to use more technologically advanced electric meters that provide greater detail and increased control over their electric usage. The system also creates new efficiencies and capabilities for transmission and distribution utilities, such as remote meter-reading and improved storm response.” (AECT)


Solar Power Generation
Describes processes that use energy from the sun to create electricity.

“There are two main types of solar energy technologies—photovoltaic (PV) and concentrating solar power (CSP). You’re likely most familiar with PV, which is utilized in panels. When the sun shines onto a solar panel, photons from the sunlight are absorbed by the cells in the panel, which creates an electric field across the layers and causes electricity to flow.

“The second technology is concentrating solar power, or CSP. It is used primarily in very large power plants and is not appropriate for residential use. This technology uses mirrors to reflect and concentrate sunlight onto receivers that collect solar energy and convert it to heat, which can then be used to produce electricity.”  (US Department of Energy)


Solar water heating
“This term comprises various technologies that convert sunlight into renewable energy that heats water using a solar thermal collector.” (PG&E) It is one of the processes solar panels use to produce electricity.


Spinning Reserves
This describes the amount of extra generation capacity not being used at a generating unit or units that can be ramped up within 10 minutes to meet unexpected peaking demand.


Zero Carbon
Power generation that does not add net carbon dioxide to the Earth’s atmosphere. Examples are wind, solar, geothermal, micro-hydro, synthetic fuels and wave energy. (, May 2013) Biofuels are not included in this category. (


Zero-emission technologies and Net-zero emissions
Zero-emission technologies do not emit greenhouse gas into the atmosphere. “In the energy sector that means using non-fossil energy sources, i.e. nuclear or renewables. However, neither is entirely carbon-free – at present we use fossil fuel to make the materials for the energy conversion technologies involved and, in the case of nuclear, to extract and process nuclear fuel. Nevertheless, they are both low-carbon options.”

“Net zero”

(emphasis added) means that carbon dioxide and other greenhouse gas emissions are reduced 100%, to zero, although some can be allowed if compensatory carbon negative processes are introduced, for example, air capture of carbon dioxide.(PhysicsWorld, Oct 2018)