In this article
A power plant needs to deliver on the day the load needs it most. That may be a hot afternoon when cooling demand is high and generation output is below its brochure rating. For many simple-cycle gas turbines, heat imposes a larger output penalty than it does on a suitably cooled reciprocating-engine plant. The useful comparison is guaranteed net power at your site—not the largest number on either brochure.
What the published comparison actually shows
Wärtsilä’s GT PRO-based manufacturer comparison gives the following 45°C / 113°F results for its engine technology and an aeroderivative turbine. The efficiency boundary is the high-voltage grid connection at sea-level pressure. These are study examples, not curves for every turbine or our individual legacy engines. [1]
| At 45°C in that study | Aeroderivative turbine example | Wärtsilä engine example |
|---|---|---|
| Output retained versus reference | About 74% | More than 98% |
| Relative efficiency reduction | More than 11% | About 4% |
These percentages are relative changes, not percentage-point changes. A hypothetical 45% efficiency falling by 4% becomes 43.2%, not 41%. Reference temperature, auxiliary loads and the measurement boundary must match before comparing equipment.
Why hotter air matters
Hotter air is less dense, reducing turbine compressor mass flow while increasing the compression work for a given air mass. Turbocharging helps engines retain output across a broader ambient range. Engines still have cooling and intake limits. Turbine inlet cooling can recover output, with additional equipment, water or electrical demand to evaluate. [1]
EPA’s CHP technology catalog supplies a separate Solar Taurus 70 example: at 100°F, output is about 80% of its 59°F reference and relative efficiency is about 8% lower. It is a different machine and comparison point, so these numbers should not be combined into the manufacturer table above. [3: EPA catalog, section 3, ambient-condition discussion]
Buy a summer operating envelope
Our recommendation is to request a single comparison sheet using the same design dry-bulb and wet-bulb temperatures, elevation, gas composition, delivery voltage and load boundary. Add the hottest expected operating condition, not just an annual average. The DOE CHP eCatalog likewise calls for final performance information reflecting site-specific fuel, ambient and thermal conditions. [2]
- State whether output is at generator terminals or after pumps, fans, gas compression, transformers and inlet cooling.
- Request net output and heat rate at several loads, with all operating units identified.
- Show cooling redundancy, fouling allowance and any water constraint.
- Separate continuous capability from a short-duration or emergency rating.
- Price any hot-weather mitigation together with its parasitic demand and maintenance.
For illustration, a site that needs 10 MW cannot treat a nominal 10 MW plant as adequate if a site-specific summer study predicts 8 MW net. Conversely, an engine plant with spare summer capacity may still need more equipment to satisfy the required outage contingency. Capacity, efficiency and redundancy answer different questions.
Apply this to an existing-core project
A proposed rehabilitation rating should be accompanied by the ambient conditions and technical scope on which it depends. Our Wärtsilä opportunities provide a starting point for that review; published modern-engine comparisons are not guarantees for a particular legacy core. Ask for the direct OEM assessment pathway, then make the agreed site performance part of the acceptance criteria.
Source review: 24 September 2026. Study results are illustrative; equipment-specific engineering governs selection.