Depends what kind of electric and how efficient the gas boiler is. If it's a modern boiler then probably about 90% efficiency, and 3400kWh of gas will give you 3060kWh of heat. And then it depends what kind of electric heating your thinking of. If it's just panel heaters then they're 100% efficient so you'd need 3060kWh of electricity to provide the same amount of heat. A heat pump is about 250% efficient, so you'd only need 1224kWh of electricity for 3060kWh of heat