As house types and heating systems can vary, it’s always best to seek the advice of a qualified professional when choosing a new boiler. A home survey by a Gas Safe registered engineer is recommended to make sure your new purchase suits you home, your needs and your budget. However, you may want an idea of boiler size options beforehand and to understand which factors to consider when choosing the right one.
The first thing to know is that boiler size relates to its output rating, not the physical size of the unit. In product descriptions you will see outputs referred to as kilowatts or ‘kW’ units which will tell you how much power a boiler can provide to your heating and hot water system.
You will then need to work out your household demands on your heating system and a useful starting point is to consider your property in the following context:

Number of radiators
Number of bathrooms
Number of people
Gas and water supply sizes

This will give you a good starting point on which boiler would best suit your needs and help you avoid the getting the wrong size. An over-sized boiler may be too powerful, leading to performance issues and wasted energy; and one that is too small may not heat your home properly.
For combi boilers, the higher the kW output, the greater the hot water they can provide. A rough rule of thumb would be a 24-28kW combi will supply a single bathroom, a 30-35kW combi will serve a bathroom and ensuite, and 35kW+ combi could serve a two bathroom property.
Is a 30kW boiler a good mid-range output and how can I make accurate comparisons between boiler makes and boiler technologies?