Gas turbines are rated at ISO conditions: 15 °C, sea level, 60% relative humidity. Hot air is less dense, so output falls as ambient temperature rises, and aeroderivatives are the most sensitive. A plant sized on ISO numbers can come up short in summer, exactly when cooling load peaks.
Ways to recover output: inlet chilling or evaporative cooling (extra capex and water use), oversizing the fleet, or choosing gas engines, which hold output better in heat.
When comparing offers, ask for output at the site’s design ambient temperature and altitude, and whether figures are gross or net of auxiliary loads (fans, pumps, gas compression).