Electric boilers work by heating water which in turn is used to heat the home via electric radiators or underfloor heating. To choose the right boiler you need to work out the size in square metres of your home and then tally this up with the right wattage and capacity.
Whilst there are plenty of calculators and instructions online about how to work this all out by measuring your room sizes, adding it all up and multiplying by a given number, you do have to be careful. The thing is, heat loss varies from home to home due to differences in insulation, and this is going to affect what you need.
How many radiators and bathrooms you have make a difference too, and you will need to take into consideration the demand for hot water. In general, the higher the KW, the better the flow rate of your hot water.
The best thing to do is speak to an electric boiler installation specialist who will advise you on precisely what boiler wattage and capacity you need for your particular home. Considering electric boilers cost anything from around £2,500 to £4,500, you really should take the time to make the right choice, and there is no substitute for professional opinion.