Lesson 4 of 8 · about 16 minutes

Capacity vs. Energy and Capacity Factor

You'll be able to

Define nameplate capacity, energy output and capacity factor
Compute a plant's expected yearly output from its capacity factor
Explain why capacity factor alone does not settle which resource is 'better' and what accredited capacity adds

A 100 MW solar farm and a 100 MW gas plant are the same size on paper and nowhere near the same thing in practice. The megawatt number is a ceiling. What a plant actually delivers over a year depends on how often it runs, and that is measured by its capacity factor.

The formula

capacity factor = actual MWh produced / (nameplate MW x hours in the period)

hours in a year = 8,760 (8,784 in a leap year)

A plant that ran at full output every hour would score 100 percent. None do. Plants stop for maintenance and refueling, the wind dies, the sun sets, water runs low, and sometimes the operator simply has cheaper options and leaves a plant idle.

Example: two plants, one size

Invented Silver Bend Solar (100 MW) produced 214,000 MWh last year. Invented Ridgeway Gas, a 100 MW combined cycle plant, produced 511,000 MWh.

Maximum possible for either: 100 MW x 8,760 h = 876,000 MWh

Silver Bend: 214,000 / 876,000 = 0.244 = 24.4%
Ridgeway:    511,000 / 876,000 = 0.583 = 58.3%

Same nameplate. Ridgeway made about 2.4 times as much energy.

What U.S. plants actually did in 2025

EIA publishes capacity factors by technology in its Electric Power Monthly, tables 6.7.A and 6.7.B. These are the annual figures for 2025, the latest full year in the tables as of October 2026, along with what 100 MW of each would produce in a year and how many average homes that energy matches (at EIA's 10,791 kWh per home).

Technology	2025 capacity factor	MWh per year from 100 MW	Average homes' yearly energy
Nuclear	91.0%	797,160	about 73,900
Geothermal	65.9%	577,284	about 53,500
Natural gas combined cycle	58.4%	511,584	about 47,400
Coal	48.7%	426,612	about 39,500
Hydroelectric (conventional)	35.3%	309,228	about 28,700
Wind	34.2%	299,592	about 27,800
Solar photovoltaic	24.4%	213,744	about 19,800
Natural gas combustion turbine	14.1%	123,516	about 11,400

Bar length is capacity factor. The scale runs from 0 at the left edge to 100 percent.

Year to year these move. Coal ran at 42.6 percent in 2024 and 48.7 percent in 2025 as higher gas prices and rising demand pulled more coal units on. Hydro swings with snowpack: it was 35.0 percent in 2023 and 41.9 percent as recently as 2018 by EIA's count. Solar and wind numbers drift with weather and with where new projects get built. Always cite the year.

Why a low number is not automatically bad

Capacity factor mixes two different causes. Some plants run less because they cannot run (no wind, no sun, low river). Others run less because nobody needs them (a peaker waiting for a heat wave, or a coal unit priced out by cheap gas). A 14 percent capacity factor is a gas turbine doing its job. A 14 percent capacity factor for a nuclear plant would be a disaster.

The bigger trap is time. Yearly energy is only part of the question. Grid planners care about the specific hours when demand is highest and supply is tightest. In many Western utilities, that is a summer evening after the sun goes down, or a still, frozen winter morning.

Example: matching energy is not matching hours

To make as much energy in a year as 100 MW of nuclear, you would need about 0.910 / 0.244 = 3.7 times the nameplate in solar, or about 373 MW. On an annual MWh basis, they are even. But at 8 p.m. in July, the solar farm makes nearly zero, while the nuclear plant is still near full output. Adding a 4-hour battery changes that picture. So does adding wind, whose output often rises at night in some places. The planning question is always "what mix covers every hour at the lowest cost and risk," never "which single plant has the biggest capacity factor."

Accredited capacity

Because of the timing problem, utilities and grid operators do not count every megawatt equally when they ask, "Do we have enough for the peak?" They assign each resource an accredited capacity, a percentage of nameplate they can count on in the riskiest hours. Many now compute it with a method called effective load carrying capability (ELCC), which runs thousands of simulated weather years and asks how much each resource actually reduces the chance of running short.

Accreditation values are set by each utility or market and change as the fleet changes. Solar's credit for the evening peak tends to fall as more solar is added, because the riskiest hour shifts later into the night. Gas plants are not counted at 100 percent either, since cold weather can freeze equipment or cut fuel supply, as lesson 8 shows. Look for the accreditation table in any integrated resource plan. It often decides which resources "win."

Common mix-ups

Claim: "Solar only works 24 percent of the time." What the evidence shows: A 24 percent capacity factor means the farm produced 24 percent of its theoretical maximum energy, not that it was on 24 percent of hours. It produces something most daylight hours, at varying levels.
Claim: "Capacity factor proves nuclear (or gas, or wind) is the best resource." What the evidence shows: Capacity factor describes output, not cost, timing value, emissions or risk. A full comparison also needs cost per MWh, accredited capacity and the hours a resource delivers.
Claim: "The plant is rated 500 MW, so it supplies 500 MW." What the evidence shows: Only at full output. Ask for its expected yearly MWh, or its capacity factor over the last three years, from EIA's plant-level data or the owner.

Reporting angle

EIA publishes plant-level monthly generation (Form EIA-923) and capacity (Form EIA-860). For any plant in your area, you can compute its real capacity factor and compare it to the national average for its type.
When a developer quotes "homes powered," ask for the expected annual MWh and the capacity factor they assumed, then redo the math yourself.
In a utility's resource plan, find the table of accredited capacity by resource type. Ask how the values were set and how they changed from the last plan.

Research angle

EIA Electric Power Monthly, Table 6.7.A: Capacity factors for utility-scale fossil fuel plants.
EIA Electric Power Monthly, Table 6.7.B: Capacity factors for wind, solar, hydro, nuclear and others.
EIA generation, capacity and sales: Capacity and generation totals to build your own ratios.
EIA FAQ on home electricity use: Household use for translating output into homes served.

Check yourself

1. A 200 MW wind farm produced 600,000 MWh last year. What was its capacity factor?

2. According to EIA data for 2025, which plant type had the highest U.S. capacity factor?

3. Why do gas combustion turbines have low capacity factors, around 14 percent in 2025?

4. Roughly how much nameplate solar capacity matches the yearly energy of 100 MW of nuclear, using 2025 EIA capacity factors?

5. What does 'accredited capacity' (or capacity credit) measure?

Terms in this lesson

Nameplate capacity: The maximum output a generator is rated to produce, in MW.
Capacity factor: Actual energy produced over a period divided by what the plant would have made running at full nameplate the whole time.
Accredited capacity: The share of a resource's nameplate that planners count on during peak or stressed hours. Also called capacity credit or ELCC-based value.
Net generation: Electricity a plant sends to the grid after subtracting what it uses itself.

Full glossary

Sources

EIA Electric Power Monthly, Table 6.7.A: Capacity factors for utility scale generators primarily using fossil fuels checked 2026-10
EIA Electric Power Monthly, Table 6.7.B: Capacity factors for utility scale generators not primarily using fossil fuels checked 2026-10
EIA Energy Explained: Electricity generation, capacity, and sales in the United States checked 2026-10
EIA FAQ: How much electricity does an American home use? checked 2026-10