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I'm looking for a vertical axis wind turbine with a power output between 3-6 kW. It needs to operate effectively at low startup speeds, suitable for off-grid installation on a rooftop. The wind conditions are variable, typically ranging from 3 m/s to 10 m/s, with an average around 4 m/s. Could you provide details on your pricing, as well as the construction materials used? I'm particularly interested in durability and efficiency, so any additional specs or features would be appreciated.