Try a mains fan heater, at 1000 watts the resistance is 57 ohms, at 2000 watts the resistance is 28 ohms and at 3000 watts the resistance is 19 ohms, so the 3000 watts setting would draw about 4 amps from 72 volt battery.
So you start at 1000 watts, then switch thru to 2000 watts, and finally...