I W = 3.412 Btu's/hr
1 Kw = 3,412 Btu's/hr
1 BTU/hr = 0.293 W
In short, if a boiler is advertised as having heat output of 14KW, then by multiplying it by 3.412 you get 47,000 BTU's which is sufficient on average to heat up to 8 standard radiators in your home. However, this does not take into account the size of rooms, insulation levels or efficiency of heating system.
BTU is short for British Thermal Unit and refers to the heating power of an appliance, measured by the amount of energy it produces in an hour.
A Watt is the Metric system for measuring heat and all boiler outputs are measured in Kilowatts(Kw)