One watt is a power of one joule per second. This is written 1 W = 1 J/s. It comes straight from the power equation, because power is energy transferred (or work done) divided by time.
So the power rating of a device tells you how many joules it transfers every second. A 60 W lamp transfers 60 J of energy each second. In 3 s it transfers 60 × 3 = 180 J. A 2000 W kettle transfers 2000 J every second.
Large powers are often given in kilowatts (kW). 1 kW = 1000 W. A 3 kW heater therefore transfers 3000 J each second. Always convert kilowatts to watts before you put a power into an equation that uses joules and seconds.