Fused,
Hugh's answer cuts to the heart of the matter, but I feel like adding that "rating" is a subjective term.
The rating of any device usually comes from the ability of its weakest part to tolerate some condition of operation. One strand of wire free in the air can take a very large current, wrap it up in plastic jacketing and then the temperature "rating" is limited by plastic melting off. Change the jacket material type and you can double the current rating of that same wire. Same wire, different rating, many reasons. And that's an extremely simple example.
Consider the case of Mariah Windpower. Up until a few years ago, their windspire was rated at 1100 watts. Then they subjected it to NREL tests and reality sunk in. They may have "rated" their system by plugging it into a VFD source and demonstrated that the boards can withstand 1100 watts in the lab. When out in the wind, rain, vibration, dust, and so on, those inverters fried. Rating under certain conditions led to optimistic projections. Since the numbers are high and that's what the sales people want to hear, that becomes the truth. The reality of the situation may be different. To their credit they seem to have gone back home and fixed a few things, but the current claim of 1200 watts is not independently substantiated, to my knowledge.
Flip the coin: Dan B just posted about bolting his recently built alternator, and included pictures of it bolted onto a car wheel to drive it for a few hours. There is a case of testing out in the cold snow, vibration, etc and Dan was able to monitor critical things like surface temperature of the stator. Now he's got empirical proof that the alternator can sustain 1.5kW, and brief periods of 2kW probably won't do damage. This data will be very valuable to him for sizing and selecting parameters for the blades to match the alternator. So.... what's the "rating" of THAT system?
