ED:
Please send your email to me to communicate extensively about it.
SOA is the transit time of the FET that limits the power dissipation in the SOA curve, the FET needs to transit fast and not to stay close to the SOA curve.
The dump controller you need for 1.2 KW needs to designed fro about 1.5 to 1.8 KW to protect you against unknown eventualities in your system.
You may have two types of controllers, the bang-bang or the proportional controller.
the best one is the proportional and just loads the generator to keep the battery at its charged voltage or at certain defined generator voltage.
Since you have a 24 volts bank, the Fet breakdown should be at least 50 volts.
The Fet current needs to be defined accordingly to the Rdson of the FET for minimum FET dissipation
Assuming 30 volts and 1,5 KW the current is 50 amps, then we need to assume the FET dissipation let's say 2 % of 1500 watts or 30 watts , which gives 30 w / 50 a = 0.6 ohms total, what about instead we say let's reduce the fet dissipation to 5 watts, then the Rdson = 5 w / 50 a = 100 milliohms = 0,1 ohms -- this require a minimum heat sink.
Now we need to find the FET, 100 to 50 volts, RDson 0,1 -- what about if 4 fets are used and each has 25 millioms RDson or even less ?
Going to www.onsemi.com I found several but one is good NTP52N10, 100 Volts, 30 milliohms, 60 amps capable, TO220 and $1.5 price.
If we used 3 in parallel the RDson would be 10 millohms at 25 Celsius and about 20 millohms at much higher temperature.
I have not tried to see how the MosFet availability is, which may require a change in the type with more commonly available.
This is the basic initial design.
Nando