A heat pump is not more efficient than a compressor. It is simply an air conditioner than can reverse the flow of the refrigerant so that the hot and cold coils can reverse their functionality. You can use the refrigerant to move heat from the inside to the outside (air conditioning) or you can use it to move heat from outside to inside (heat).
The only brag that heat pumps have in terms of efficiency is that when used to heat a house, they are much more efficient than the resistive technology of a hair dryer, which is basically what all previous forms of electric heat amount to. An electric furnace was basically a huge hair dryer. Electric baseboards use the same hair dryer technology without a fan.
Any direct burning of fuel will always be much more efficient at heating than a heat pump, and air conditioning with a heat pump is EXACTLY as efficient as air conditioning with an air conditioner. Heating in a car is provided from using some of the excess heat from burning gasoline in the engine.
More recently, there has been electronic means of functioning as a heat pump without refrigerant gases or compressors. This was developed by NASA and is extremely efficient in terms of low consumption of electricity, but it also doesn't pack much of a thermal punch. It warms things and cools them. It doesn't heat them or chill them.
Meanwhile, 3 amps at 300 volts is 900 watts. That's not trivial for a battery pack of this capacity. At 144 volts, that's more than 6 amps, and the battery pack is not much over 6 amp hours. You'd go from fully charged to dead batteries in less than an hour.