A 120V 15A circuit will provide about 1.4kW (120*(0.8*15)) or 1.9kW for a 20A circuit
Most J1772 style L2 chargers are 7.6kW (240*(0.8*40)) rarely more, sometimes less
Tesla HWPC units are configurable based on the input supply circuit rating.
However, the point is well made. There are plenty of block heater circuits out there and indeed my wife uses one to charge the car while she's at work. It provides enough power to cover the energy needed to get home (about 54km).
