I really wonder how a heat pump can achieve so much efficiency gain. Heat of electric resistance is about 99% energy efficient: almost all energy is converted into heat, nothing else.

A resistive heater directly converts electricity to heat, whereas a heat pump instead moves heat, from the outside to inside of your car. The heat your car gains is reflected by the heat the outside loses. In that way, they can be 300-400% “efficient”, because we do not care about the outside air around the car getting a little bit colder.

> whereas a heat pump instead moves heat, from the outside to inside of your car.

I completely forgot that the energy of the outside air is used, that explains it of course.

It can be that efficient when set to some point that would cook you. For low human temperatures that we call comfortable, it is not so efficient.

What is the electrical energy converted into at lower temperatures if not heat?

It just goes back to the battery to complete the circuit. Heat is created by resistance and if the energy going through the heater isn’t being resisted enough, there is no heat, just wasted energy. Most of these heaters regulate temperature by turning off/on/off and it’s the coming up to full resistance that is when the energy is “wasted” and doing it at human temperatures is a lot of off/on cycles. Also, it’s worth pointing out that not 100% of the energy is converted to heat, or you’d have a gigaton bomb instead of a heater.