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IF the heater runs continually. Your maths seems to assume that the heater runs the whole time the car is in use. Not so - heaters have thermostats, the heater will run until cabin temp reach the selected temp and then cycle off. So although the heatpump uses less power than the resistance heater, you need a run profile (outside temp, selected heater temp, + radiant input from sunshine) to calculate the real load and therefore the actual resulting reduction in range.