You can calculate watts from joules and seconds, but you can't convert joules to watts, since joule and watt units represent different quantities.
The power P in watts (W) is equal to the energy E in joules (J), divided by the time period t in seconds (s):
P(W) = E(J) / t(s)
watt = joule / second
W = J / s
What is the power consumption of an electrical circuit that has energy consumption of 90 joules for time duration of 3 seconds?
P(W) = 90J / 3s = 30W