What Is Heat Rate? – The Efficiency Number Behind Every Gas Plant
Every gas plant brochure, utility filing and on-site power proposal eventually lists a number like 8,500 Btu/kWh. It looks like an odd unit, energy divided by energy, and plenty of smart people nod along without being sure whether higher or lower is good. Yet heat rate decides more about a gas plant's operating cost than almost any other figure. What is heat rate, and why does it matter so much?
Heat rate is the amount of fuel energy a power plant burns to produce one kilowatt-hour of electricity, usually stated in British thermal units per kWh (Btu/kWh), and lower is better. Since one kWh of electricity equals 3,412 Btu, a perfect plant would have a heat rate of 3,412.
Real plants burn more, so dividing 3,412 by the heat rate gives efficiency: 7,500 Btu/kWh is about 45% efficient, and 10,500 is about 33%. Heat rate multiplied by the fuel price gives the fuel cost of every MWh, which is why buyers, lenders and grid operators watch it closely.
What Does Heat Rate Measure?
Heat rate measures how much fuel a plant needs to make a unit of electricity. The U.S. Energy Information Administration defines it in its FAQ on power plant efficiency as the Btu of fuel consumed per net kWh generated. It is the inverse of efficiency, expressed in units that plant operators and fuel buyers find practical.
Why not just say "40% efficient"? Because heat rate plugs straight into fuel math. Gas is bought in million Btu (MMBtu). If you know the heat rate, you know how many MMBtu each MWh will burn without any extra conversion. An 8,500 Btu/kWh plant burns 8.5 MMBtu per MWh. That is a number a fuel buyer can use immediately.
How Do You Convert Heat Rate to Efficiency?
Divide 3,412 by the heat rate. The 3,412 comes from the energy content of one kWh expressed in Btu.
- 6,500 Btu/kWh: 3,412 ÷ 6,500 = 52.5%
- 8,500 Btu/kWh: 3,412 ÷ 8,500 = 40.1%
- 10,000 Btu/kWh: 3,412 ÷ 10,000 = 34.1%
To go the other way, divide 3,412 by the efficiency. A 45% efficient plant has a heat rate of 3,412 ÷ 0.45 ≈ 7,580 Btu/kWh.
The chart below plots that relationship. Notice the curve flattens toward the high heat rates on the right: shaving 1,000 Btu/kWh off an already efficient plant gains more efficiency points than the same cut on an inefficient one, but in fuel terms every 1,000 Btu/kWh saves exactly the same 1 MMBtu per MWh.
Efficiency = 3,412 Btu per kWh divided by heat rate
Model: efficiency = 3,412 ÷ heat rate (Btu/kWh). Labels are heat rates in Btu/kWh.
View the data
| Efficiency | |
|---|---|
| 6,000 | 56.9% |
| 7,000 | 48.7% |
| 8,000 | 42.7% |
| 9,000 | 37.9% |
| 10,000 | 34.1% |
| 11,000 | 31.0% |
| 12,000 | 28.4% |
What Is a Good Heat Rate?
It depends on the type of plant. A heat rate that is excellent for a small engine would be poor for a big combined-cycle unit. EIA reported that in 2015 the average US combined-cycle plant heat rate was 7,340 Btu/kWh, while simple-cycle gas turbines averaged 9,788 Btu/kWh. That gap of nearly 2,500 Btu/kWh comes from reusing exhaust heat, which is the whole idea behind a combined cycle plant: the steam turbine makes extra power without burning extra gas.
| Plant type | Typical heat rate range (Btu/kWh) | Approximate efficiency |
|---|---|---|
| Modern large combined cycle | About 6,000 to 7,000 | About 49% to 57% |
| Fleet-average combined cycle | About 7,000 to 7,500 | About 45% to 49% |
| Large reciprocating engines | About 7,200 to 9,000 | About 38% to 47% |
| Aeroderivative turbines, simple cycle | About 8,300 to 9,500 | About 36% to 41% |
| Older or smaller simple-cycle turbines | About 10,000 to 13,000 | About 26% to 34% |
Treat these as broad ranges, not specs. Manufacturer figures are usually on a lower heating value basis at ideal conditions, while fleet statistics reflect real operation. Always compare like with like.
Why HHV vs LHV and Net vs Gross Matter
Two quiet details can swing a heat rate by 10% or more.
The first is the heating value basis. Burning natural gas produces water vapor. Higher heating value (HHV) counts the energy you could recover by condensing that vapor; lower heating value (LHV) does not. For natural gas, HHV is about 11% higher than LHV. Equipment makers usually quote LHV heat rates, which look better. Gas is bought and sold on HHV. A turbine at 8,500 Btu/kWh LHV is roughly 9,400 Btu/kWh HHV.
The second is net versus gross. Gross output is what the generator makes. Net output subtracts the plant's own use: fans, pumps, gas compressors and controls. Heat rate on a net basis is higher, and it is the honest number for what you can actually deliver.
How Is Heat Rate Measured?
On paper, heat rate is fuel in divided by power out. In the field, getting both numbers right takes care. A plant measures fuel flow with a gas meter and fuel energy content with a gas chromatograph or lab samples, then measures net electrical output at the revenue meter. Divide one by the other over a set period and you have a measured heat rate.
For contract guarantees, the measurement follows a formal performance test. Gas turbines and whole plants are commonly tested under ASME Performance Test Codes, such as PTC 22 for gas turbines and PTC 46 for overall plant performance. Testers run the plant steady at set loads, record ambient temperature, pressure and humidity, and then correct the results back to the reference conditions in the contract. Without those corrections, a test on a cool morning would flatter the plant and a test on a hot afternoon would punish it.
Operators also track heat rate continuously. A slow upward drift is often the first sign that a compressor needs washing or an engine needs attention.
What Makes Heat Rate Go Up?
Heat rate climbs, meaning efficiency falls, whenever a plant runs away from its ideal conditions. Common causes:
- Part load: most turbines and engines are most efficient near full output. Running a turbine at half load can raise its heat rate substantially.
- Hot weather and altitude: thinner air reduces turbine output and efficiency.
- Wear and fouling: dirty compressors, worn seals and fouled heat exchangers creep heat rate up over time. Compressor washing and overhauls bring some of it back.
- Frequent starts: fuel burned during startup produces little or no electricity.
- Parasitic loads: a fuel gas compressor or emissions equipment adds plant load and raises net heat rate.
How Heat Rate Turns Into Fuel and Dollars
The basic formula is simple: fuel cost per MWh = heat rate (in MMBtu per MWh) × gas price (in $ per MMBtu). Here is an illustrative example:
- Heat rate 8,500 Btu/kWh = 8.5 MMBtu per MWh.
- Gas price $3.00 per MMBtu.
- Fuel cost = 8.5 × $3.00 = $25.50 per MWh.
Now see what a 1,000 Btu/kWh difference means for a 50 MW plant running all year. Annual output is 50 MW × 8,760 hours = 438,000 MWh. Each MWh saves 1 MMBtu, so the better plant saves 438,000 MMBtu a year. At $3.00 per MMBtu, that is about $1.3 million every year, for the life of the plant.
To turn those MMBtu into gas volume, you need the gas heating value. Pipeline gas averages about 1,036 Btu per cubic foot, so 1 Mcf (one thousand cubic feet) carries roughly 1.036 MMBtu. Our guide on Mcf to MMBtu conversion covers the details. Or try the calculator below: enter plant output, heat rate and gas heat content, and it returns daily fuel in MMBtu and Mcf.
What Is a Market or Implied Heat Rate?
Traders and analysts also use heat rate in a different sense. The implied or market heat rate is the power price divided by the gas price. If power sells for $40 per MWh and gas costs $3.00 per MMBtu, the market heat rate is 40 ÷ 3 ≈ 13.3 MMBtu per MWh, or 13,300 Btu/kWh.
That number tells you which plants can make money on fuel alone. Any plant with a heat rate below 13,300 Btu/kWh covers its fuel cost at those prices; one above it does not. The gap between the power price and a plant's fuel cost is called the spark spread, and it is the starting point for judging whether a gas plant will run.
Heat Rate Is Your Fuel Bill in Disguise
Heat rate is the Btu of fuel a plant burns per kWh, and lower is better. Divide 3,412 by it to get efficiency, multiply it by the gas price to get fuel cost per MWh, and always check whether the number is HHV or LHV, net or gross, and at what load and temperature.
When you compare generation options, ask every vendor for guaranteed net heat rates at your site conditions and at several load points. Then run the dollars. A small difference in heat rate is a large difference in money over twenty years.