So you want a dump load that will only kick in when the system voltage approaches (rises to) the voltage at which the regulator built into the turbine will otherwise take over and start wasting power itself. Unless I'm mistaken, you can trigger the dumping action on nothing more than system voltage. Charging or not, lights on or not, if the voltage is going up to the range where the turbine takes over, you want to preemptively use your new load. I think this is a relatively easy task except for the currents involved. It'd be good if somebody with some experience would chime in here. Just to get you thinking, here's a circuit I just posted for somebody else, showing how excess solar panel power can be shunted to ground if the system voltage rises above a user-settable target. This circuit is fine for 50mA, not 15A. The voltage is set by the zener diode D2 and tweaked by resistors R2 and R3. It's set up to put ~0.65v at the transistor's base - turning on the shunt to ground - when current starts flowing through the zener, which only happens when the battery voltage gets too high, above the zener voltage plus the drop across the 2 resistors.