Getting off topic .... again ......
An electric resistance heater (e.g. storage heater, immersion heater) is by definition 100% efficient, all the energy that goes into them comes out as heat. The problem is that it costs you north of 20p/kWh on the typical electricity tariff today, compared to around 5p/kWh for a domestic A rated gas boiler.
A "heat pump" uses the electricty to take heat from a cold source and "pump" it up to a warm dump, and under the right circumstances can achieve a performance ratio greater than one, e.g it uses 1kWh of energy but moves 2kWh, it isn't creating heat it's just moving it. In that case each kWh of heat into your sitting room will have cost you 0.5kWh of electricity. Efficient but still expensive....... and the electricity will have come from a gas fired power station, so you may as well burn the gas in the house unless the coefficient of performance is better than 2.5