Cheap energy, firm capacity, flexibility: which plant does which job?

Sort the technologies by the jobs they do

Lesson 6 of Energy Systems, Step by Step: meet the three jobs a power system needs done, compare nine technologies by their capacity factor and firm share, sort them by job, and weigh both sides of a contested pairing.

The idea in one table: three jobs, and who does each well

Job The question Does it well
Cheap energy Many MWh a year at a low running cost? Wind, solar, nuclear, hydro, geothermal
Firm capacity There in the riskiest hours, whatever the weather? Nuclear, gas plants, geothermal, demand response
Flexibility Up or down quickly on request? Gas peakers, batteries, reservoir hydro

With JavaScript on, you work through this step by step: nine technology cards, a chart of two numbers, a sort and a contested question.

Capacity factors: U.S. 2025 averages, average output ÷ size, from EIA's Electric Power Monthly, Tables 6.07.A and 6.07.B (released 23 October 2026).

Firm shares: PJM's ELCC class ratings for the 2028/2029 auction, one market and one auction. They rate technology classes; a single plant's accredited MW can differ.

Also: battery round trip, EIA (2019 data); demand response, FERC (2024 data).

What you just did, as diagrams

Cheap energy
Fill the year with many MWh at a low running cost; capacity factor measures the MWh per MW, running cost (lesson 7) the rest.
Firm capacity
Be there in the riskiest hours; firm share measures it.
Flexibility
Change output quickly on request; ramp rate and start-up time measure it.
Two different numbers
Gas peakers averaged 13.6% of their size in 2025, yet PJM counts 67% of their MW at the peak; solar averaged 24.0% but counts 7%.
No single winner
No technology does all three jobs best, so grids combine them.
Go deeper · AnalystHow a grid measures firm share

A firm share, which PJM calls an ELCC class rating, asks: how much steady, always-available capacity would keep the grid exactly as reliable as this plant does?

Planners answer it by simulating many weather years with random plant outages.

  • It depends on the grid. The numbers here are PJM’s for its 2028/29 auction; another region, or another year, gives others.
  • It falls as more of the same is added. The tenth solar farm shifts the riskiest hours into the evening, so it counts for less than the first, though not always.
  • Longer storage counts more. PJM counts 59% of a 4-hour battery’s MW, 68% at 6 hours, 71% at 8 and 78% at 10.
  • Classes, not plants. These are ratings for technology classes; PJM can accredit a single plant differently.

Analyst lesson A5 runs this simulation for the Plains grid.

Go deeper · ModelerThe three jobs as three kinds of rule in a planning model

A planning model turns the three jobs into rules. In words:

  • Decisions. How many MW of each technology to build, and how much each one makes every hour.
  • Goal. The lowest total cost of building and running them.
  • Rule 1, energy. Supply equals demand in every hour.
  • Rule 2, firm capacity. The firm MW add up to the year’s peak demand plus a safety margin.
  • Rule 3, flexibility. No plant changes its output faster than its ramp limit.

Rule 2 in symbols, with \(C_g\) the MW of technology \(g\), \(f_g\) its firm share and \(m\) the margin (for example 0.15):

\[ \textstyle\sum_g f_g\, C_g \;\ge\; (1 + m)\, D_\text{peak} \]

Lesson 10 teaches how to write rules like these down; lesson 13 asks a solver for Prairie Town’s least-cost mix.

Go deeperWhat this lesson leaves out
  • Costs: what each job costs to buy is lesson 7.
  • Wires: a firm plant behind a full power line cannot help the town that needs it.
  • Rules: markets reward the three jobs in different ways (lesson 8, and Analyst lesson A3).
  • Averages: every number is a fleet average or one market's rating; single plants differ.

Check yourself

Myth or fact? “A plant's capacity factor tells you how much it counts at the peak.”

  • Myth
  • Fact
Answer

Myth. Solar's capacity factor was 24.0% in 2025, but PJM counts 7% of its MW at the peak; gas peakers averaged 13.6% but count 67%.

Myth or fact? “A battery is a source of energy.”

  • Myth
  • Fact
Answer

Myth. It moves energy in time and loses some on the way: the U.S. battery fleet gave back 82% of what it stored (2019 data).

Myth or fact? “A gas peaker with a capacity factor of only 13.6% is not doing a useful job.”

  • Myth
  • Fact
Answer

Myth. A gas peaker's job is to be there at the peak and to move fast; whether that is the best way to do the job is a question of cost (lesson 7).

Which number tells you most about a plant's firm capacity?

  • Its capacity factor
  • Its firm share at the riskiest hours
  • Its size in MW
  • Its running cost
Answer

Its firm share: the part of its MW a grid can count on in the riskiest hours. Size alone overstates it; capacity factor measures energy per MW.

Which job does a 4-hour battery do best?

  • Cheap energy
  • Firm capacity, for as long as any peak lasts
  • Flexibility
Answer

Flexibility: it switches between charging and discharging almost at once. It helps with firm capacity only for as long as its stored energy lasts (PJM's 4-hour rating: 59%).

Numbers, sources and what’s next

Numbers to remember

Number What it means Source
58.7% Average capacity factor of U.S. gas combined-cycle plants in 2025. EIA, Electric Power Monthly, Table 6.07.A · 2025 data (final), released October 2026
13.6% Average capacity factor of U.S. gas turbines (peakers) in 2025. EIA, Electric Power Monthly, Table 6.07.A · 2025 data (final), released October 2026
7% PJM's class rating for fixed-tilt solar: the share of its MW counted toward peak needs (gas peaker 67%, 4-hour battery 59%). PJM, ELCC Class Ratings for the 2028/2029 Base Residual Auction · read November 2026
82% Share of stored electricity the U.S. battery fleet gave back. EIA, Today in Energy · data for 2019
33,272 MW Demand response enrolled in the seven U.S. wholesale markets, about 6.5% of their peak demand. FERC staff, 2025 Assessment of Demand Response · data for 2024

Next on the starter path

Lesson 7 · What does electricity cost to make?

In the real world

Regions reward these services differently: through energy prices, capacity payments or planning requirements, and payments for reserves and fast response. Which technology wins each depends on the region's rules and weather.

Sources and further reading

Spotted an error, or stuck? Write to hlli@ksu.edu. Corrections are made in place and dated here.

Text licensed CC BY 4.0, code MIT: teachers and students may reuse and adapt it, with credit to Honglin Li.