400W is 400W - so yes, any electric appliance which generates heat is as efficient as a resistive electric heater is. Whatever tiny amounts of power are leaking

400W is 400W - so yes, any electric appliance which generates heat is as efficient as a resistive electric heater is. Whatever tiny amounts of power are leaking as light or sound or electromagnetic waves are miniscule compared to the heat. However, we have better ways of heating. An AC unit running in heat mode, or an air source heat pump, will move 4x the heat into your room than it takes in electricity to run. So although running a plasma is equal to running a resistive baseboard heater, if you want the best bang for your kWh you should get a minisplit air conditioner or air source heat pump.