The large difference in power is probably caused by the different voltages. The rotor has an optimum cubic line and the generator must be loaded such that the Pmech-n curve of the generator is lying close to the optimum cubic line of the rotor. The Pmech- curve shifts to the right if the voltage is higher (see report KD 78). If the generator is loaded by only a 24 battery, the voltage at high power can become very high if there is no battery charge controller with dump load which limits the voltage. The voltage becomes that high that the Pmech-n curve has been shifted that much to the right that it lies close enough to the optimum cubic line to realize an acceptable matching and so an acceptable Cp and power. However, if the inverter is coupled to the battery, the charging voltage is that low that the distance in between the Pmech-n curve of the generator and the optimum cubic line of the rotor becomes too large. So the generator will be loaded too strong to get an acceptable matching and so an acceptable Cp and power. I think that the power will go up strongly if you use a 36 V battery, at least if your inverter can be used for a nominal battery voltage of 36 V.