Perhaps you should consider building your own. You could find a known 12 volt design that works for the 12.5 foot blades. You would use the same magnets and rotor and just wind the stator coils to the same size using wire having one half the diameter of the original design. Why would this work? Well, the turbine can only put out so some much power at each wind speed. At 48 volts to get the same power into the battery pack you need 1/4 the current. This means you would need sixteen times the resistance in your coils to control the current. Also, to get the 48 volts you need 4 times the number of windings. Well, it just happens that using a wire 1/2 the diameter accomplishes both of these requirements. One half the diameter implies 1/4 the cross sectional area, which means 4 times the resistance per unit length. It also means 1/4 the volume per unit length. The total length of wire goes up by a factor of four giving the necessary number of turns. The resistance has gone up by a factor of sixteen. This will not be a perfect match as the diode voltage drops will change very little. In others words the cut-in RPM will be reduced slightly. It may be necessary to add some extra resistance to the battery to prevent stall.
