Automotive and some marine alternators typically have an efficiency curve with a peak of 60%.
Image from http://www.intechopen.com
Whereas alternators to other types of applications reach 96% of efficiency (excited rotor type).
What are the factors that makes then so inefficient compared to other applications? Is pole-number, air-gap? Could the efficiency be improved with some modifications for stationary use or the unique way is maintain the speed at the peak of efficiency?
@olinlathrop suggested somethings, among that the environment the alternator works (temperature, vibrations, dust) means their robustness can be a trade off to the efficiency.
I agree in some points, although there's no objective answer, good sealed bearings would not decrease the efficiency that much, so I think temperature can be one of the factors as they are small and works near a combustion motor, even trough have their own forced ventilation.
Anyway where is an alternator with stated 80% efficiency at 450A 24A! That's ~10Kw. http://www.emp-corp.com/media/MarketingMaterial/Power450/SpecificationSheets/Power450.pdf
The difference is that this alternator uses magnetics instead of electro-magnets in the rotor. Anyway, the ~95% efficiency alternator product sheet I read is a excited rotor one.