I have been involved with the Provens since they started and before in the early 1990s. I worked closely with them in the mid nineties to get their first model (2.5 aka 3.2 kW) working right. It's a simple and robust design. It's actually the change in blade angle (pitch) rather than coning that controls the speed and this is achieved by the action of centrifugal force against a spring.
It's basically a very solid and simple machine, but it typically goes through a beta test period of about 7 years before a product is good to buy. The 6 kW is nice now although they did change the blades on both the 2.5 and the 6 which threw the speed control system into chaos, and caused a few burned stators. Recently they found they could push the 2.5 to 3.2 kW for grid tie so they changed the name. They used to offer a 600 watt machine for about 12 years that was actually vary nice, but they have just dropped it, and decided not to supply spares which strikes me as incredibly irresponsible behaviour.
The 15 kW is pretty early in this process and most of the time they take it off the market and ignore the existing customers who have turbines that do not work. So far as I know they are not available right now. But the status changes fairly often. I would not order a 15 at this time, but the smaller ones are (or used to be) pretty solid if you can afford them.
The big problem these days is getting to speak with anyone in Proven who can help you, since they only deal through re-sellers and they are mostly too busy to keep them informed, and hardly anyone in Proven seems to understand how the turbines work. They got big financial backing and too many new staff.
