Buy or build a CHP system. Water cooled diesel engine driving a generator. If you can, do it with belt drive to mains alternator, add a 24V alternator to directly charge batteries, and run the cooling water through your radiators and to heat domestic water. Belt drive also means that you can run the engine well below rated RPM (you will probably need an electronic speed controller though). An exhaust cooler will further reduce waste heat, as would a turbine on the exhaust to drive a 2nd 24V alternator. I intend to drive a brushless DC motor of the type used in hobby drones*. I have something similar, but I want to add an electric heater in the water circulation to get the temperature up more quickly. The more heat that you can use, the more efficient the whole system is. I estimate that I am getting 90% efficiency.
* these motors are rated by power and KV, KV is the RPM per volt (or volts per RPM !). In this scenario, RPM will depend largely on how much current is allowed to flow. Make sure that you get the permanent magnet variant for use as an alternator. You will also need a 3-phase bridge rectifier that will handle the current. The power that these motors can produce is phenomenal.
APS 4260S Sensored Outrunner brushless motor 100KV 1200W