Heat Rate & Efficiency Converter — kJ/kWh, BTU/kWh, %

Value to convert

Given as

Fuel price (per MMBTU)

Plant output for daily cost (MW)

Quantity	Value
Efficiency	–
Heat rate	–
Heat rate	–
Fuel energy per MWh generated	–
Fuel cost per MWh	–
Fuel cost per day at the stated output	–

The Calculation, Explained

Definitions

Heat rate is the fuel energy that must be supplied to generate one unit of electrical energy, measured at a defined boundary. Efficiency is the electrical energy out divided by the fuel energy in. They are reciprocals of each other, scaled by the energy content of one kilowatt-hour:

1 kWh = 3,600 kJ = 3,412.14 BTU exactly (by definition of the units)

η = 3,600 / HR_kJ/kWh | HR_kJ/kWh = 3,600 / η | HR_BTU/kWh = HR_kJ/kWh / 1.05506

Lower heat rate means a better plant. The fuel-cost line follows directly: one MWh at heat rate HR consumes HR × 1,000 kJ = HR / 1055.06 MMBTU, and multiplying by the fuel price gives the fuel cost of that MWh — the number that appears, plant by plant, in tariff determinations.

Assumptions and applicability

Same basis, or the comparison is void. A heat rate may be stated on the fuel's LHV (net) or HHV (gross) basis. For natural gas the two differ by about 10% — a plant quoted at 6,800 BTU/kWh (HHV) and another at 6,150 (LHV) may be the same plant. Every figure from this calculator inherits the basis of the figure you entered; it converts units, not bases. To change basis: HR_LHV = HR_HHV × (LHV/HHV of the fuel).
Same boundary. Gross heat rate (at the generator terminals) and net heat rate (after auxiliary consumption) differ by the plant's house load, typically 2–4% for a combined cycle plant.
The cost figures assume the stated fuel price is the delivered price per MMBTU of fuel energy on the same basis as the heat rate.

Worked example (the defaults above)

Heat rate 9,850 kJ/kWh: efficiency = 3,600 / 9,850 = 36.55%. In imperial units: 9,850 / 1.05506 = 9,336 BTU/kWh. Fuel per MWh = 9,850 / 1055.06 = 9.336 MMBTU, so at a fuel price of 10.00 per MMBTU the fuel cost is 93.36 per MWh — and an 850 MW plant running flat out spends ≈ 1.90 million per day on fuel alone. This is why a 1% heat-rate improvement is worth fighting for in a tariff hearing.

Practitioner's note. When two heat rates disagree, ask three questions before believing either: LHV or HHV? Gross or net? Corrected to reference conditions, or as-run? Most "miracle" efficiencies dissolve under one of the three.

References

ASME PTC 46 — Overall Plant Performance (definitions of heat rate and test boundary); unit relations per the SI Brochure (BIPM) and NIST SP 811.

Frequently Asked Questions

How do you convert heat rate to efficiency?

Efficiency = 3,600 divided by the heat rate in kJ/kWh, because one kilowatt-hour is exactly 3,600 kJ. A heat rate of 9,850 kJ/kWh is 36.5% efficiency; 7,200 kJ/kWh is 50%.

What is a good heat rate for a combined cycle power plant?

Modern large combined cycle plants achieve roughly 5,900 to 6,600 kJ/kWh on an LHV basis (55 to 61% efficiency). Older or smaller plants commonly run 7,200 to 8,600 kJ/kWh, and simple-cycle gas turbines 9,500 to 11,500 kJ/kWh. Always check the basis before comparing: LHV versus HHV alone shifts the number by about 10% on natural gas.

Why can two reports show different heat rates for the same plant?

Usually one of three reasons: the fuel basis differs (LHV vs HHV), the boundary differs (gross output at the generator vs net after auxiliary loads), or one figure is corrected to reference conditions and the other is as-run. Ask all three questions before believing either number.

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