The system as a whole is far from 100% efficient. Once you add transmission losses and generation losses, electric resistive heating will be probably below 25% for the system as a whole, as most of the heat will be getting made outside of the building that's being heated.
Gas heating would be far more efficient as a system, since the losses in shipping the gas to your house to burn will be less than shipping the gas to the power station, burning it there to make heat, turning that into electricity, transmitting the electricity, then turning what's left back to heat in the building.