The power required by any fluid mover is the product of the pressure rise times the volumetric flowrate. So, for a piece of air moving equipment operating at a higher altitude, the density is of primary interest. Density will affect pressure rise.
Trane shows Performance Adjustment Factors for its packaged roof top units. They refer to AGA Std 221.30 - 1964, Part VI, 6.12. I don't know if this standard is based on empirical data or a formula.
Looking at the factors it appears that compressor power increases about 1% per 1000 foot elevation while cooling output is decreasing by about 1% per 1000 foot elevation. The factors go up to 7,000 feet. If the evaporator fan speed is not increased by adjusting the pulleys, the brake horsepower of the fan motor drops off as the ratio of the higher air density over standard air density.
You can download Trane packaged rooftop data from the internet.
Altitude and Power Consumption
Re: Altitude and Power Consumption in AC Units
