You can calculate joules from kilowatts and seconds, but you can't convert kilowatts to joules since kilowatt and joule units represent different quantities.
The energy E in joules (J) is equal to 1000 times the power P in kilowatts (kW), times the time period t in seconds (s):
E(J) = 1000 × P(kW) × t(s)
joules = 1000 × kilowatts × seconds
J = 1000 × kW×s
What is the energy consumption of an electrical circuit that has power consumption of 3 kilowatts for time duration of 3 seconds?
E(J) = 1000 × 3kW × 3s = 9000J