That depends on the power rating (in kW) of the boiler. As a rule of thumb, one radiator requires 1.5kW of energy, but not all boiler types are equal. If you’re looking at a combi boiler, which supplies hot water when it’s needed, look at 24-27kW models to heat homes of up to 10 radiators, with 28-34kW suitable for up to 15 radiators, and 35-40kW models recommended for homes with up to 20 radiators. System and heat-only boilers, which only need to heat hot water 2-3 times a day, can be smaller – so 12-15kW are suited for homes with up to 10 radiators, 18-24kW for 10-15 radiators, and 30-35kW for homes with 15-20 radiators.