Lesson 3 of 8 · about 15 minutes

Turning gas into power: turbines, combined cycle and engines

You'll be able to

Describe simple-cycle combustion turbines, combined-cycle plants and reciprocating engines
Convert a heat rate into efficiency and fuel cost with real numbers
Explain why combined-cycle plants run often and peakers run rarely
Compare start and ramp speeds and say what they mean for a grid with lots of solar and wind

Every gas power plant does the same basic thing: burn methane, turn heat into spinning motion, spin a generator. What differs is how much of the heat becomes electricity, how quickly the plant can start and how often it runs. Those differences decide what a plant costs, what it emits and what job it does on the grid. When a utility says "we're adding 500 megawatts of gas," the first question to ask is what kind.

Simple-cycle combustion turbines

A simple-cycle combustion turbine works like a jet engine bolted to the ground. A compressor squeezes air, fuel burns in the combustor, and the hot, expanding gas spins turbine blades on a shaft connected to a generator. The exhaust, often around 1,000 degrees F, goes straight up the stack.

Two families exist. Aeroderivatives, adapted from aircraft engines, are lighter and start quickly. Heavy-duty "frame" turbines are bigger and built for industrial duty. Either way, throwing all that exhaust heat away limits efficiency. EIA's tested average heat rate for US gas turbines was 10,999 Btu/kWh in 2024.

These units are the classic peaker: cheap to build per kilowatt, expensive to run, called on for the hottest afternoons and coldest mornings. EIA's capacity factor for the US gas turbine fleet was 13.9% in 2024. Most hours they sit.

Combined-cycle plants

A combined-cycle plant starts with the same gas turbine but catches the exhaust in a heat recovery steam generator, a big boiler with no burner of its own (some have supplemental "duct" burners). The steam drives a second turbine. Typical arrangements are "1x1" (one gas turbine, one steam turbine) or "2x1" (two gas turbines feeding one steam turbine).

In a combined-cycle plant, roughly a third of the power comes from the steam turbine, using heat a simple-cycle unit would waste.

The payoff is big. EIA's tested average heat rate for US combined-cycle units was 7,548 Btu/kWh in 2024, and the newest large units are rated lower still. That efficiency is why combined-cycle plants are the workhorses: the US fleet ran at a 60.5% capacity factor in 2024, higher than coal's 42.6%.

Reciprocating engines

A reciprocating engine plant uses big piston engines, often 5 to 20 MW each, lined up in a hall. They are less efficient than a big combined-cycle plant but better than a simple-cycle turbine (EIA's 2024 tested average for gas internal combustion units was 8,924 Btu/kWh). Their selling points are modularity and speed. Twelve 18 MW engines can be run as one, three or twelve units, each at its best efficiency point, and engines typically reach full load in minutes. They also tolerate frequent starts and stops better than most turbines. That has made them popular for grid firming and, lately, for on-site power at data centers that cannot wait years for a grid hookup.

Heat rate, efficiency and fuel cost: the math

One kilowatt-hour of electricity contains 3,412 Btu of energy. So:

Efficiency = 3,412 / heat rate
Fuel cost per MWh = (heat rate / 1,000) x gas price per MMBtu

The heat rate divided by 1,000 gives MMBtu per MWh, because 1 MWh is 1,000 kWh and 1 MMBtu is 1,000,000 Btu.

Unit type (EIA 2024 tested average)	Heat rate, Btu/kWh	Efficiency	Fuel cost at $3.50/MMBtu	2024 capacity factor
Combined cycle	7,548	45.2%	$26.42/MWh	60.5%
Internal combustion (engines)	8,924	38.2%	$31.23/MWh	18.0%
Gas steam boiler (older plants)	10,337	33.0%	$36.18/MWh	19.9%
Combustion turbine (simple cycle)	10,999	31.0%	$38.50/MWh	13.9%
Coal steam (for comparison)	10,018	34.1%	depends on coal price	42.6%

Example: same gas, two plants, one hot afternoon

Gas at the plant costs $3.50 per MMBtu. Grid operators need 100 MW for four hours, so 400 MWh.

Combined cycle at 7,548 Btu/kWh: 400 MWh x 7.548 MMBtu/MWh = 3,019 MMBtu. Cost: 3,019 x $3.50 = $10,567.

Simple-cycle peaker at 10,999 Btu/kWh: 400 x 10.999 = 4,400 MMBtu. Cost: 4,400 x $3.50 = $15,400.

The peaker burns 46% more gas for the same power, and emits 46% more CO2. So why build peakers? Because a combined-cycle plant that isn't already running may need hours to come online, and building one costs much more per kilowatt. For a few hundred hours a year, the cheap-to-build, fast-to-start unit wins the math.

Now raise gas to $12, as can happen in a regional cold snap. The peaker's fuel cost becomes 10.999 x $12 = $132 per MWh. Gas price swings hit inefficient units hardest.

How fast each one moves

Start time and ramp rate are separate speeds. Start time is how long from cold to full power. Ramp rate is how fast output can change once the unit is running. EIA's 2020 analysis of its generator survey found that about a quarter of US power plants can go from shutdown to full output within an hour. Most combined-cycle units reported 1 to 12 hours, with some under an hour. Nuclear almost always needs more than 12 hours. Hydro is fastest, under 10 minutes for most units.

Technology	Typical cold start to full load	Ramping once running	Usual role
Aeroderivative turbine	About 5 to 10 minutes (manufacturer ratings)	Very fast	Peaker, reserves
Frame combustion turbine	About 10 to 30 minutes	Fast	Peaker
Reciprocating engines	About 5 minutes or less (manufacturer ratings)	Very fast, good at partial load	Firming, data center on-site power
Combined cycle	Under 1 hour to 12 hours (EIA survey); "fast-start" designs quicker	Moderate	Baseload and load-following
Coal steam	Many hours	Slow	Baseload (declining)

The minute figures for turbines and engines come from manufacturer specifications and vary by model and by whether the unit is kept warm. Treat them as ranges, not promises, and ask a utility for its own unit's tested start time.

Speed matters more every year. On a grid with lots of solar, demand left for other plants drops at midday and then climbs steeply around sunset. Something has to cover that evening ramp. Batteries now cover much of it in places like California and Texas. Fast gas units cover much of the rest. Lesson 4 picks up that role.

Turbine supply in 2026

New gas plants run into a manufacturing bottleneck. GE Vernova, one of three big heavy-duty turbine makers along with Siemens Energy and Mitsubishi Power, reported 116 GW of gas equipment backlog and slot reservations at the end of June 2026 (53 GW in firm orders, 63 GW in reservations), up from 100 GW three months earlier. It shipped 3 GW in that quarter, aimed for an output rate of about 20 GW a year by the third quarter of 2026 and planned to reach 24 GW a year in 2028. At that rate, a new order placed today waits years for delivery. Lesson 4 looks at why demand jumped.

Common mix-ups

Claim: A gas plant's megawatt rating tells you how much power it makes. What the evidence shows: Rating is maximum output. A 400 MW peaker at a 14% capacity factor makes about 490,000 MWh a year. A 400 MW combined-cycle plant at 60% makes about 2.1 million MWh. Always ask for expected annual generation.
Claim: All gas plants have the same emissions per kWh. What the evidence shows: CO2 per kWh scales with heat rate. A peaker emits roughly 45% more CO2 per kWh than a modern combined-cycle plant burning the same gas.
Claim: Gas plants can start instantly. What the evidence shows: Simple-cycle units and engines start in minutes. Many combined-cycle plants take hours from cold. "Gas is flexible" depends on which gas.

Reporting angle

EIA Form 860 lists every US generator with its technology, capacity and in-service year. Form 923 gives monthly fuel use and generation. Divide fuel by generation to estimate a plant's real heat rate.
For a proposed plant, ask for: technology type, number of units, expected heat rate, expected annual capacity factor, start time and who supplies the turbines or engines and when.
Ask whether the air permit caps annual operating hours. Many peaker permits do, and a cap tells you how the owner really expects to run it.
If a utility says a plant will "support renewables," ask how many starts per year it expects and what the permit allows.

Research angle

Check yourself

1. A plant has a heat rate of 7,548 Btu/kWh. About what is its efficiency?

2. What makes a combined-cycle plant more efficient than a simple-cycle turbine?

3. Per EIA's Electric Power Annual, about what capacity factor did US gas combustion turbines (simple cycle) run at in 2024?

4. Which statement about start times is supported by EIA?

5. Gas costs $4.00 per MMBtu. A peaker has a heat rate of 11,000 Btu/kWh. What is its fuel cost per MWh?

6. Why do some data center developers choose reciprocating engines?

Terms in this lesson

Heat rate: Btu of fuel burned per kWh of electricity produced. Lower is more efficient.
Simple-cycle combustion turbine: A jet-engine-like gas turbine that spins a generator and sends its hot exhaust up the stack. Fast but less efficient.
Combined cycle (NGCC): A gas turbine plus a heat recovery steam generator and steam turbine that make extra power from the exhaust heat.
Reciprocating engine: A large piston engine, similar in principle to a car engine, burning gas to drive a generator.
Capacity factor: Actual output over a period divided by what the unit would make running at full power the whole time.
Aeroderivative: A combustion turbine adapted from an aircraft jet engine. Lighter, faster to start, often used as a peaker.

Full glossary