3kW is what an imersion heater uses, until the cylinder is hot and the thermostat clicks off.
It is also what a typical electric heater, washing machine, tumble-drier or kettle uses, so if you run any one of these for an hour, it will use 3kWh.
Look at your bill, you are probably paying about 14p per kWh
Lighting, TV etc use next to nothing, especially if you have energy-saving lamps.