You can calculate watt-hour from watts and hours, but you can't convert watts to watt-hour, since watts and watt-hour units represent different quantities.
The energy E in watt-hour (Wh) is equal to the power P in watts (W), times the time period t in hours (h):
E(Wh) = P(W) × t(h)
watt-hour = watts × hours
Wh = W × h
What is the energy consumption in watt-hour when the power consumption is 5 watts for time duration of 3 hours?
E = 5W × 3h = 15 Wh